From D&D Wiki
Prestige classes (PrC) are a form of multiclassing in 3.5e. They can represent many things such as membership of a certain society or possession of unique skill. Unlike the basic classes, characters must meet Requirements before they can take their first level of a prestige class. The rules for level advancement apply to this system, meaning the first step of advancement is always choosing a class. If a character does not meet the Requirements for a prestige class before that first step, that character cannot take the first level of that prestige class. Taking a prestige class does not incur the experience point penalties normally associated with multiclassing.