Prestige classes are 'special roles' for PC's (and NPC's) that go beyond the bounds of standard classes. As prestige classes are more 'focused', one could view them as less versatile, but definitely more powerful.
The special roles offer access to abilities and powers otherwise inaccessible to PC's and focus them in specific and interesting directions. Prestige classes are for those who wish to focus on a specific area for their pc, rather than continue along the 'center path' of the normal classes.
Acquiring a prestige class requires a player to meet certain certain base requirements first, and therefore are not available to low-level characters. Additionally, beyond the game 'mechanic' requirements, the character must meet non rule-related requirements in-game, such as group membership fees, special training, quests, and so forth.
The DM must always be consulted before a player can take any prestige classes. It is advised players discuss with their DM well ahead of meeting the qualifications, lest they waste much time and character development in pursuit of a Prestige Class the DM just won't let them have.
Listed here are the prestige classes currently developed, and the rule-related requirements. Some of the Prestige classes listed are marked as 'unavailable to PC's in the Stormkeep campaign.' This is true for all evil Prestige Classes. They are, however, on the list due to the fact that NPC's of these types are found within the world, and some players may have heard dark whispers of their foul deeds.
Please consult your DM if you are interested in pursuing any of these classes or in assisting in the creation of a new prestige class in order to focus your player in a specific direction which you have in mind.
Note that, unlike standard multiclassing, acquiring a prestige class does not incur the experience point penalty of un-balanced class levels.