No, archetypes are not subclasses. They’re a whole system of character modifications, most of which can be taken by any character as long as they meet the prerequisites. They usually modify some base element of your class (eg the Flexible Spellcaster archetype changes how casters select their spells, use their spell slots, and how many spells they get). There are a subset of archetypes (Class Archetypes) which are locked to specific classes, and which more deeply alter the class’s base abilities. The changes can be quite significant. This is where the presteige classes are rearing their heads.