Here’s a good start:
http://www.google.com/webmasters/4.html
Disregard the part about using Google’s link tool–use Yahoo’s instead. (Google’s not displaying good backlinks right now.) To check backlinks with Yahoo, go to yahoo and type linkdomain:http://www.yoururl.com.
The theory behind linking is that if a lot of pages link to a site, then it’s popular and relevant. Especially if the pages linking to that site use the search term as the anchor text for the link. And you get even more points for linking to a page that’s on-topic.
I’ve read it somewhere, probably on Google, but link to sites that you think your visitors will find useful.