I looked around, and while the regular wiki is informative to a degree it isn't concise. The most useful discussion of abdl terms, including sissy and babyfur I found is at Babyfur - WikiFur, the furry encyclopedia. I'll reprint them here
There are a few sub-groups within the babyfur community which are often described with the following terminology :
- Babyfur : Used to describe the group as a whole, however, specifically it refers to someone who roleplays as younger characters—up to the human equivalent of toddler age. Baby paraphernalia is likely to be involved for this group and roleplay speech is often either in gibberish or very basic speech.
- Lilfur (littlefur, kidfur) : This is a person who roleplays as a child character. Characters identified as lilfurs tend to be between their toddler and teen ages. Diapers and other baby paraphernalia may or may not be involved. Role played speech is usually more refined at this point, but still somewhat juvenile.
- Diaperfur : This is a person who roleplays as an adult character who still either requires or chooses to wear diapers. Role played speech and actions are consistent with an adult. The person may also roleplay an adult character who has mentally regressed back to a younger state, but still has an adult physical form.
- Sissyfur : This is commonly misunderstood as a person who roleplays a male character, either adult or child, who is forced to dress and act as a young female character. Truthfully, a sissyfur is anyone, male or female, who enjoys regression, leaning toward a very feminized and docile or submissive look and attitude.
- Caregiver/Caretaker : As the name suggests these are the people who roleplay characters who give care, a 'mommy' or 'daddy' figure for other babyfurs.
Many people don't identify with one single group, but can change roles depending on the situation and who they are interacting with. It's also not uncommon for complex lineages (for example siblings, parents, etc.) to be set up among other babyfurs so that roles can be defined.
Definition of AB/DLs
These are general non-furry terms but are useful to understand in this context.
- Adult Baby (AB) : A person that enjoys regressing back to being a baby. Beyond regressing using physical or environmental changes (such as diapers, clothing, and settings), they also mentally regress, taking the mentality of a baby. These are the equivalent to 'babyfurs' or 'lilfurs' when crossed with furry. Teenagers who are Adult Babies will sometimes refer to themselves as Teen Babies (TBs).
- Diaper Lover (DL) : A person feels a strong desire to wear or use diapers, usually without physiological necessity. These people, unlike AB's, will usually not regress physically (adopting a baby form) or mentally. These tend to be the equivalent of 'diaperfurs' in the babyfur community. For some DLs, but not all of them, there is a fetish connection to the diapers. They will sometimes take the submissive role to a carer in sexual play, and the carer will take a dominant role.
- Infantilist : Essentially, infantilism regards any person who enjoys regressing to the mental age of an infant or toddler, and may or may not involve diapers and other baby paraphernalia.
I'll add "Littles" to that list. Littles are, in general persons, whether furry or not, who voluntarily regress. They may roleplay or imagine themselves to be any age younger than in real life. There exist littles who don't care for diapers, who prefer simply to be a small child.
We often get abdl newcomers who have known and accepted being so for different lengths of time but only recently have discovered the rest of us and decided to join in the fun. If there are other terms they should know about, please do mention them here.