Monks, Sorcerers, and Barbarians are classes from the D&D 3rd Edition rules. They were included in the BG2 crpg (which implemented AD&D 2nd Edition rules), because pnp 3E rules were being published around the same time.
AD&D 2E had several "core" rule books, such as the "Player's Handbook", "Dungeon Master's Guide", and "Monsterous Manual".
AD&D 2E also had many "supplemental" rule books, such as "The Complete Fighter's Handbook", "The Complete Priest's Handbook", and so on. These books each contain many, many optional kits for the fighter class, priest class, and so on. Bioware selected 3 kits from each of the main 8 classes to implement into BG2.
There were also several "Player's Option" and "Dungeon Master Option" rule books published to expand AD&D 2E. I think the HLAs from BG2 were inspired from these.
For a not-too-shabby list of AD&D (pre-3E) material, check out this website:
TSR Archive: Advanced Dungeons & Dragons
(There are probably better organized, more complete lists out there somewhere.)
When pnp 2E materials were being published by TSR in the 1990's, they also created different fantasy settings. Each "universe" featured a different flavor, such as:
Greyhawk (typical fantasy, one of the first settings, originally started with AD&D 1st Edition)
Forgotten Realms (typical fantasy, TSR's largest most detailed setting)
Ravenloft (gothic horror setting, my absolute favorite)
Spelljammer (wizards in space setting)
Oriental Adventures (kung-fu setting)
Al-Quadim (genies in bottles setting)
Birthright (ruling a kingdom setting)
Planescape (bizzarro setting (?), joining all the different 2E universes (i.e. Planes of Existence))
...and many others. Check it out.