alt131 - 5:42 pm on Sep 25, 2012 (gmt 0)
As rainborick says, the recommendations are really helpful, but they can be challenging to read at first. The Web Education Community Group [w3.org] has some tutorials that break the information down and give examples for you to try. Positioning is split into static and relative positioning [w3.org], and absolute and fixed [w3.org].
In terms of your question about when each would be used, that really does depend on what you are trying to achieve.