Lee Hopkins and Trevor Cook have written the second edition of their Social Media eBook. If you feel uncertain about the SM (no, not that one!) world, then this will help. It’s a 57-page pdf but it’s an easy and informative read.
Don’t be put off by its Australianness or its datedness in the early parts, it probably needs a slight update when it comes to mentions of things like Writely, which was renamed last October. The value of this .pdf eBook is that it’s running you through the principles of the new web world.
The wiki bit is the least insightful by these two excellent writers, but they compensate by examining real projects so you should get an idea of their relevance to you.
Nothing is beyond reach: Facebook, Twitter, Jaiku, Second Life … Offhand, I couldn’t think of anything significant that they left out. (The providers of the services that weren’t mentioned will probably disagree. They can chuck in their comments on this blog if they like.)
Take a look. It costs nothing apart from half an hour or so of your time.