There is a total of 27 classes that you may pick to play in the world of Derlith.
They are listed here, but can also be found in the Book of Knowledge, if you'd like a more customizable list.
These classes each have their own special set skills. Click on a link for their class page, which gives an introduction to the class and an overview of their abilities. Please note that while there is only one Cleric, Deathknight, and Paladin class, they all are able to worship different deities which grants them with different sets of skills.
Multiclass characters are a common thing in standard D&D and thus were ported into Realms. Here, they are special hybrid classes that have a combination of skills from the two classes they are mixed as. In some cases, certain abilities from the two classes might not even be available.
The trade-off is that their skills for BOTH classes will never be as good as those of a pure classed character. Furthermore, there is a in any experience you gain for levels 1-30, which is then reduced to for levels 31-40. These classes should only be chosen by experienced players, for they are hard to stat as well as require more time than a normal class would. Though, high leveled multiclass characters can be considered as well-rounded with many cool skills to choose from.
All multiclass characters train at the location of their first class. Stat and save adjustments per level has been customized for each of these combinations.