Archetypes is a framework designed to ease the building of applications for Plone and CMF
Dexterity is a content type framework for CMF applications, with particular emphasis on Plone
"Dexterity arguably has more better documentation than archetypes see plone.org products dexterity as well as my book professional plone 4 development"
from question "Archetypes vs. Dexterity for new content types and new field types"
"The good news is that dexterity content types are more lightweight than archetypes content types and doing raw listing by iterating over folder.contentitems in your template should not be that expensive"
from question "How to get the image of Dexterity content types and show in a batched listing view?"
"Variety of supporting widgets is probably the single area in which archetypes is still stronger than dexterity"
from question "Is there a Field/Widget for dexterity that does the same as Products.UserField does for Archetypes?"