A class is a set of skills. These skills are also referred to as guild-skills, because you must join a guild in order to gain a class, although as you will read later on, you may, after a time, have a class without being in the associated guild. Examples of classes are Monk (Sentaari), Druid, and Zealot (Daru).
Once you join a guild, you'll automatically gain the class of that guild, but will only be an apprentice in that class. Your apprenticeship will end when you first gain guild rank 3. Once you have finished your apprenticeship, your class is permanent. Even if you are demoted back below guild rank 3, you will stay a full member of the class.
What follows is a list of classes, the skills involved with that class, and the guilds that can give you that class.