TechSoup (the technology place for nonprofits) has an an excellent Web page summarizing Web 2.0 platforms and applications, titled "What Is Web 2.0 Anyway?" "New media" resources described in a very clear and readable fashion are:
Blogging
RSS
Tagging and Social Bookmarking
Widgets and AJAX Applications
The paragraph on RSS is probably the most succinct and understandable entry on this technology I've read so far.
The article ends with links to TechSoup resources on each of these topics. It's a great place to start if you have very little knowledge (but lots of interest!)
