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, and Kinoko Juice. 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, CP/Fairyland (very limited).
Covers a lot of topics! Events, pages for each doll sculpt with info on one-off models released. A bit of info on FCS, not really any photos though. Good place to waste time looking at the limited doll you want!
English FCS guide. Based off of Angel Den with updated information. Lots of photos so you can dream about going to Japan and ordering an FCS doll.
The original FCS guide in English. Stopped being updated in Sep 2012. No photos anymore.
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. A recast master list exists. If the doll you are looking for is not on this list, it is unlikely a recast version exists. 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.
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 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.
These include Volks Dollfie Dream and Obitsu mostly. Some smaller companies make dolls using Obitsu frames including Azone, VisuaDoll, Yamato VMF50, Pink Drops, Angel Philia. CP/Fairyland has started their own line of vinyl dolls called AniFee. Granado has made vinyl versions of their large boys called Vindoll.
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.
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 into 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.
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.
BJDs are grouped into 3 size groups: tinies, minis, and largies.
Eyes are usually made out of acrylic, glass, silicone, or resin. Resin is regarded as the best right now (my info is 10 years old, sorry). 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. The best place to buy resin anime eyes is in person at a Dollpa, unfortunately.
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. 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. Most companies require EMS shipping, so keep that in mind with your budget. It's usually best 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 would not suggest buying a doll off AliExpress or Taobao, unless you are looking for a recast. A few Chinese companies do have legitimate store fronts on these sites though! Be very cautious when buying off ebay as well.
Some Japanese hashtags and my attempt at translation. I don't speak any Japanese btw. Try searching on Flickr, Twitter, and Instagram. Many dealers and artists also have their own hashtag that buyers will post under. Many Super Dollfie sculpts also have their own hashtag such as #SD_f_63 or #SD63番 (numbered sculpts) or #SDリズ (sculpts with names). There's a few variations on the format though.
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: