An archetype is a character concept more specific and involved than a theme, but not as comprehensive or broad as a class. Each archetype represents a significant divergence from the abilities of a typical member of the core classes. Archetypes provide an additional layer of control for players who want to fine-tune their character’s advancement. An archetype generally grants abilities that aren’t otherwise available to characters through a class, or it may grant easier access to a specific set of appropriate abilities. In general, these abilities aren’t tied to the background of any one core class or theme and aren’t available to characters via other sources.
So Starfinder is using archetypes as sets of class features that represent a concept that isn’t tied to any one class, but shouldn’t be available to every character simply by taking a set of feats. Unlike Pathfinder archetypes, which must be designed for a single specific character class it can be added to, Starfinder archetypes are designed to be applicable to any character class. Each class has specific class features it gives up at set levels (2nd, 4th, 6th, 9th, 12th, and 18th) if the archetype provides an alternative class feature at that level (though not every archetype will need to have alternate class features at all those levels). An archetype thus becomes part of the class you attach it to, and that class loses a pre-determined ability at the levels the archetype grants you an alternative. For example, the Starfinder Core Rulebook has two archetypes—the phrenic adept and the Starfinder forerunner. The phrenic adept has alternate class features at every level it can—2nd, 4th, 6th, 9th, 12th, and 18th—while the Starfinder forerunner only provides alternate class features at 2nd, 4th, and 6th.
You must decide to add an archetype to a class when you gain the first class level at which the archetype has an alternate class feature. For example, the Starfinder forerunner archetype grants its first alternate class feature (trained for trouble) at 2nd level. So if you take a second level of a class, and decide to add the Starfinder forerunner archetype, you gain trained for trouble as a class feature. What you give up depends on what class you took—a mechanic that gains the archetype would not receive the mechanic trick normally available at 2nd level, while a mystic would gain 1 fewer spells known for the highest level of spell it can cast, and so on. Every class outlines what class feature it forgoes if it has an archetype added that has an alternate class feature at that level. Each class you take can have at most one archetype attached to it, and you cannot attach the same archetype to more than one class if you multiclass.
Archetypes are designed to represent sets of abilities you could reasonably gain regardless of your class or theme, but that involve some sort of special circumstance or training that makes them bad choices as feats. The phrenic adept assumes you have gained some innate psychic powers, as a result of natural mutation, specialized training, exposure to an ill-understood force such as an alien artifact, and so on. The Starfinder forerunner represents the result of special training given to members of the Starfinder Society who do advanced work to prepare for Starfinder expeditions, and often serve as guides on such expeditions. Neither is designed to be as all-encompassing as a full character class, but both represent more new abilities than should be contained in 1 or 2 feats. As the Starfinder RPG grows, we’ll be looking for places it makes sense for a new option to be more than a feat or theme but less than a class, and then using archetypes to make those available to characters of any class.
As a preview of what archetype features look like, here is the ability gained by the phrenic adept at 4th level.
Phrenic Defense (Ex): Your psychic powers give you additional defenses against mental attacks. The first time you fail a saving throw against a spell or effect with the emotion, fear, mind-affecting, or pain descriptor (see page 269), you can spend 1 Resolve Point as a reaction to immediately reroll the failed saving throw.
Even if the second saving throw fails, your stronger defenses might reduce the effect of the spell or ability. If the spell or ability deals damage, reduce the damage done by an amount equal to your class level. If the spell or ability has a duration of 2 rounds or longer, reduce its duration by half.