Content types enable you to manage the metadata and behaviors of a document or item type in a centralized, reusable way. The following links are a good starting point to get more information about how Content Types works in SharePoint.
- "What are Content Types, Anyway" by Andrew May
- "Introduction to Content Types" on MSDN
- Content Type Technical Poster (pdf) by www.officezealot.com
- Plan content types on TechNet
- "Building Custom Content Types in WSS" on MSDN/Visual How Tos
- Content Types and Columns (How Do I..) on MSDN/SDK
- Content Types on MSDN/SDK