Prestige Classes are schools of learning open only to a select few. Each class has a set of requirements required for transition, which is done at mortal level 50. You retain all of the skills from your low-mort levels, although in some cases, skills contrary to the new class's teachings may be removed. After level 50, you level as your new class in all ways, including gains, new skills and spells, and so on. You retain the same Prime Requisite as your original class, and the same stats and attributes you had before.
For a breakdown of the basic requirements to become a prestige class, see the Prestige Requirements.