I know a thing or two about dolls so please take some of my knowledge to start your own doll journey. Ara has also created a BIG BJD FAQ. She knows a lot, so read her page too!
These dolls are made out of polyurethane resin (plastic) and are strung together using elastic. Resin is known to yellow over time and with UV exposure so these dolls are best kept out of the sun, although they eventually will turn a yellowy color as they age. Many of these dolls will be more realistically styled compared to vinyl dolls. Brands usually tend to stick to a style, so if you like a doll from a certain company, you'll likely find more you're interested in there. The largest English forum to discuss these dolls is Den of Angels.
Many companies produce these dolls. Japanese brands include Volks, Alchemic Labo, Kinoko Juice, Aoi Tuki. Korean brands include Fairyland, Iplehouse, Soom, Luts. Chinese brands include Angell Studio, Dollchateau, Ringdoll. A few companies make resin dolls in an anime style: Luts, 2D Doll, SQLab, Myou Doll, Puyoodoll, Isladolls, Xagadoll, Aimerai.
Doll kits were once popular as a way to save money, but you're unlikely to run into one these days without searching. The keyword is 組立キット for assembly kit btw if you are looking!
I have a whole page on Volks Super Dollfie, if you are curious about those in particular.
Recast resin dolls are extremely common. These are dolls that have been created without permission of the original company. Criminals make a new mold of the doll and sell it as their own at a lower price. Be careful not to be scammed when buying new or used dolls. Many sellers will say in the listing that their doll is a recast, but some sellers are either dishonest or don't know their item is a fake. Ask around if you are nervous about a particular listing. A lot of pro-artist people, as well as DoA moderators, would be happy to help. Edit: DoA moderators may or may not answer you in a reasonable amount of time, so it's best to make a friend. I have a page on recast identification tips for more information.
Vinyl dolls are generally made out of a plastic skeleton frame covered by soft vinyl. They're also quite a bit lighter than resin dolls and less fragile. Vinyl is not known to yellow much over its lifetime, but has a lot of issues with staining. Take care when using dark fabrics and wig caps on your dolls. The majority of vinyl dolls are styled more "anime" than resin dolls. The largest English forum to discuss these dolls is Doll Dreaming. Major BJD forum DoA considers Dollfie Dream and Obitsu dolls grandfathered in as BJDs.
A long term issue with these dolls is that frames can break and the replacement parts may become hard to find once the doll line is discontinued. It may take a bit of effort to find the parts secondhand and you could be better off replacing the body instead.
Dollfie Dream is the most popular line. Volks creates limited models as well as standard models. Limited models tend to be more expensive and are collaborations to recreate anime or game characters. Customized heads by popular artists can also reach similar prices as limited dolls. Size of the doll mostly doesn't affect the price of the doll for these.
Obitsu is a Japanese plastic manufacturing company that has been around for just about forever. They offer a wide range of skeleton based vinyl bodies. There's many head options, both in anime and a more realistic styles. These dolls are much cheaper than their Volks counterparts and replacement parts are readily available. Parabox designs outer vinyl parts to go over the Obitsu frame. I'm not entirely sure of the relationship between these two companies, but Parabox is often referred to as "Parabox Obitsu". Parabox has an online shop that ships internationally, unlike Obitsu.
VisuaDoll makes dolls based off the obitsu frame. There's around 4 faces to choose from and the faces are closer to BJD style than anime.
Azone has Pure Neemo (1/6), Pico Neemo (1/12), Another Realistic Characters (1/3), and Iris Collect (1/3) lines. Both Pure Neemo and Pico Neemo use a non Obitsu frame, so they are not grandfathered as BJDs. These two lines also have painted on eyes and rooted hair, making them less customizable. Another Realistic Characters and Iris Collect use Obitsu bodies with vinyl heads with removable eyes with a few exceptions.
VMF50, Pink Drops, and Angel Philia are on the Obitsu frame, but are usually very sexualized. There's some relationship between these three that I don't quite understand yet.
Aya Doll is another company that makes mini sized dolls based off the Obitsu frame. Their faces almost resemble an ichimatsu doll.
Dollce sells soft vinyl heads that fit on bodies from other companies. They also sell some Obitsu 11 based dolls.
Fairyland has started their AniFee line, which are made of soft vinyl on a plastic frame. So far, reviews on the body have not been great. Hopefully in a few years they'll have improved their product.
Granado has made vinyl versions of their large boys called Vindoll. They are significanly cheaper and lighter than their resin counterparts.
Japanese company Dollbot has appeared in 2022 and has released a vinyl doll on their own frame.
Dollzone/Doll Chateau sister company TinyFox produces "MJD" which cannot be disassembled past the head or repaired. I don't think we can consider these BJD by any definition, but they are styled similarly.
Bootleg Dollfie Dreams exist. So far, Snow Miku and possibly Saber have popped up. Casting vinyl has a huge upfront cost unlike resin, so these took a lot longer to show up and aren't plentiful.
Kikipop is not a BJD, but I want to add there is a fake kikipop. You would have to be blind to mistake one for the real deal. Absolutely terrifying.
BJDs are grouped into 3 size groups: tinies, minis, and largies.
I'm the boss of this guide so I'm adding big babies.
Eyes are usually made out of acrylic, glass, silicone, or resin. Acrylic eyes generally don't have the clarity of the other eyes, and often have cheaply printed irises. Glass eyes can vary in quality. You can find them from $10 to $200+ for some dealer made eyes. The higher quality ones have more detail in the iris in comparison to the lower quality ones. Silicone eyes have mostly gone out of style, but are squishy! Soom and Masterpiece make them. Don't use silicone ear plugs to install these in your doll!! Resin/urethane eyes used to be rare and relatively expensive, but now are probably the most common eyes. They can be made at home with not much of an investment in equipments so there are tons of artists out there creating them. There's tons of different styles and varying levels of realism. As always, resin does have a yellowing problem. These eyes may need to be replaced after a few years. Custom animetic eyes are almost always in resin. The best place to buy resin anime eyes is in person at a Dollpa, unfortunately, but more places online have started to offer them in recent years.
Eyes can be held in the head with hot glue, poster tack, or silicone ear plugs. I recommend poster tack for resin dolls and silicone ear plugs for vinyl dolls. I've never been able to get poster tack to stick to the smooth insides of a vinyl head. If you can find beige poster tack, that's awesome! I haven't found any yet so please excuse the blue that can sneak into my dolls' eyes.
Wig sizes are measured by the circumference of the dolls head in inches. Heat resistant fibers look less shiny and can be heat styled. Volks has started recommending the Tangle Teezer brush for styling, but a normal comb or toothbrush can be used. Avoid using wig brushes on real hair because of oils in human hair.
My advice when choosing a first doll is to wait until you find the one you're in love with. Jumping on the first doll you find before you develop your tastes usually results in not being satisfied with it. Do not buy the first doll you can afford. The best way to find the doll you want is to look at a ton of dolls. In addition, the cheapest doll isn't likely to be your favorite either. Saving up for the one you love is usually your best option.
Please do your research once you're in love! Some of these companies put up beautiful photos where they use angles and lighting magic, but in actuality, the doll only is half as pretty in person. Others have terrible quality control, terrible shipping times (a whole year!), bad reputations, etc. A few companies have disappeared off the face of the earth, hopefully fulfilling their final orders first though. Find out what others think of this company and look at their photos of the doll sculpt. Consider if the body is pleasing to you as well. Think about if you prefer aesthetics or posability. Some bodies have both, some bodies have one, some bodies have neither.
If you find the company has a less than good reputation, consider buying the doll secondhand. This way you know the flaws of the doll you're getting and get to skip out on dealing with a bad company.
When buying a new resin doll, expect long wait times. The dolls are made to order and companies will not let you cancel the order after submitted. (EDIT: Refunding orders after long wait times seems to be the trend these days.) Most companies require EMS shipping, so keep that in mind with your budget. DHL has become a more common option now as well, but both are very expensive for a large doll box halfway across the world. It's usually fastest to order from the companies themselves when buying new, but if you'd like to save on shipping, dealers such as Denver Doll Emporium submit all doll orders from a time period to the company, recieve all the dolls in the order from the company, then ship out each doll individually from their location (which is hopefully closer to you).
I highly suggest researching the wait time of current orders. If the wait time is over 6 months, proceed cautiously. If the wait time is over a year, I would suggest looking for secondhand or at other companies. A few companies have dropped off the face of the earth without fulfilling orders and you want to have PayPal's protection.
Here are a few links to get you started. There are many many other companies out there.
I would not suggest buying a doll off AliExpress or Taobao, unless you are looking for a recast or know what you are doing. A few Chinese companies do have legitimate store fronts on Taobao though! Etsy (AKA AliExpress 2.0) is almost always recasts, with the exception of the Kinoko Juice shop and a few other "small artists". Half the Etsy clothes/accessory sellers out of China also sell recasts. Do your research on everything there.
eBay is now a complete mess and BJD are no longer in their separate category. I have not been successful in creating the perfect search to only show BJD. Be very cautious when buying off ebay. Vague listings are not your friend and ebay is filled with new and used recasts as well as "Night Lolita" style dolls which are not BJDs. There are legit secondhand sellers out there and it may be worth keeping an eye on for you.
This may be one for the hot takes page, but try to avoid Facebook and Instagram, at least at first. The lack of organized feedback system makes it very easy for scammers to get away with tricks and people are not held accountable. Scam alerts are quickly washed away by multitudes of posts overwhelming them. Don't be a pushover when dealing with people online. Not everyone is a good person.
Many people send out their doll heads to be painted by artists or even paint them themselves. Buying a pre-customized head is also an option. Here is a guide on how to do faceups yourself from Den of Angels.
A few warnings:
Magic erasers (melamine sponges) are your friend! They'll remove just about any surface stain, faceup, or dirt off your resin doll. They will remove sealant so keep that in mind! Don't attempt to use the eraser on your doll's face unless you want the faceup removed. Or the body blushing! A quick and easy cleaning method is to remove your doll's head and run them in the dishwasher.
Vinyl dolls stain easily when exposed to dyes in fabrics, but these stains can often be removed with chemicals such as Remove-Zit and Volks Dream Rescue.
I can't consider myself a great doll photographer by any means, but here's some things to pay attention to when shooting your dolls.
First make sure the eyes are in straight, then worry about if your doll looks dead or not. The top of the pupil should be very near the upper eyelid, if not covered by it. Different glances are more unsettling than others. Usually eye contact and far off glances work.
Would a human ever stand in the pose your doll is in? Proper girls in skirts don't manspread!
Don't just plop the wig on their head and call it good. Is the forehead too big? Too small? A bit of brushing goes a long way even if you dont fix every flyaway.
I've been using these terms so long I can't remember them not making sense. I may not be the best person to write this since I can't tell what's weird anymore.
 Disclaimer: I take no responsibilty for any damage caused by the dollfiewasher.