From publisher blurb:
What is a Prestige Archetype?
A prestige archetype is a prestige class turned into a normal 20-level class, similar to an archetype or alternate class.
The goal when making these prestige archetypes has been to capture the flavor of the prestige class rather than to make an exact copy of how it would normally be. Attack bonuses, hit dice, class skills, spells, and class features have been unified and spread evenly over levels. This means that most prestige archetypes have their prestige abilities spread over the entire 20 level spread. Unlike how prestige classes normally work, these archetypes have a few complete class abilities rather than many abilities limited to a low level. In some cases, you get the choice of one of several class abilities rather than stymied versions of all the abilities.
A prestige archetype generally has elements of one or more regular character classes, to represent how prestige prerequisites were met and character development after the prestige class has ended. Sometimes there is more than one class that naturally leads itself to a particular prestige class, which can lead to several different prestige archetypes for different combinations. A few are not built on a regular class, instead expanding the idea behind the prestige class to a full 20 levels.