The recommended way to develop new custom content-types is by using Dexterity. You'll find the full manual here.
Up until Plone 4.1, the standard way was to use Archetypes. You can still develop using Archetypes, and they will remain functional. For any new development, Dexterity is strongly recommended.
The Archetypes reference manual will remain available at the Plone 4 documentention