A class is a profession or vocation for a character. It determines what he or she is able to do: combat training, magical ability, skills, and more. Selection of a class for a character is typically based on the core concept for the character. The eleven basic selections can approximate almost any character concept, and often several options are available. For example, both paladins and clerics are excellent at hunting down undead monsters.
There are the eleven base classes (also called core classes) from which all characters must choose to start their adventuring careers. These classes are barbarian, bard, cleric, druid, fighter, monk, paladin, ranger, rogue, sorcerer, and wizard. Each class has a range of abilities that make it substantially different from the other classes. These abilities are an integral part of a character's class, and mastering them will aid the player. Some classes have alignment restrictions. For example, a paladin must be of lawful good alignment. If the character's actions shift this alignment, then the character can no longer gain paladin levels. Similarly, a neutral good character cannot gain paladin class levels, since the character is not of the required alignment.
Shadows of Undrentide added a new option for character development called prestige classes. Prestige classes are like regular classes, except that they have requirements that must be met before one can attain the class. These powerful classes grant characters unique new abilities unavailable to the base classes. Some classes may require a certain number of ranks, others may require the ability to cast arcane or divine spells, certain alignments, feats, or races. A prestige class cannot be chosen for a starting character, it must be earned through careful advancement; be aware of the prerequisites before planning which prestige class to take.
Shadows of Undrentide introduced five prestige classes, specifically arcane archer, assassin, blackguard, harper scout, and shadowdancer. Hordes of the Underdark introduced an additional six prestige classes, specifically champion of Torm, dwarven defender, pale master, red dragon disciple, shifter, and weapon master. With patch 1.69 purple dragon knight became available.
With the release of Hordes of the Underdark, a character can achieve epic class levels. Core classes become epic at class level 21, while prestige classes become epic at class level 11. Epic class levels cannot be obtained until character level 21. An epic class receives additional class-specific benefits (as described in each class' description) when the character levels up.
Player characters begin with one base class but, as the character gains experience points, the player may choose to take a second or third class, which may be either base or prestige. Characters may have a maximum of three classes. Characters are expected to keep their base classes as close to the same level as possible, or be subject to a multiclass penalty to earned experience points. However, an exception to this rule is made for favored classes and prestige classes.
According to BioWare, the three class limit is mostly to keep the graphical interface clean, easy to understand, and easy to use. There was also a consideration that taking more than three base classes (the only classes available when the three class limit was adopted) has a tendency to introduce a rather severe multiclass penalty, particularly among players not familiar with the third edition Dungeons & Dragons rules.