Intro to Sabermetrics

Michael Jong, of Marlin Maniac and Beyond the Boxscore, has written quite the nice set of articles outlining some of the major concepts of modern day Sabermetrics at FanHuddle:

Glossary: part one, two and three.
Runs created
The value of walks
The “international currency” of baseball (runs)
Linear weights

He also has several in depth player profiles, which are also very informative.

If you are interesting in learning about Sabermetrics, from the bottom up, this is a very good place to start.