An archetype is a domain level model of data points and data groups relating to a topic. See the Archetype specifications page for a technical description and specifications. The Archetype Technology Overview is useful for a business level overview.
If you are clinical and/or a health informatician, you can find hundreds of openEHR archetypes at the Clinical Knowledge Manager (CKM) .
If you want to build archetypes, see the Online Archetype Designer.