The recommended way to develop new custom content types is by using :term:`Dexterity`. You'll find the full manual here.
.. toctree:: :maxdepth: 1 /external/plone.app.dexterity/docs/index
Up until Plone 4.1, the standard way was to use :term:`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