I actually did a lot of research on this question a few months ago and came across a lovely page explaining it. I'll have to try to find it for you!
A lot of the confusion comes from translating British English patterns to American English. Britain apparently has 3 or 4 different ways to do yarn overs depending on the end result desired.
Bah I really can't give the explanation justice. I'll have to look for the page I mentioned! I'll edit when I find it!
Finally! I must have really been digging deep when I came across it the first time. Took me 2 hours to find it the second time!
All you could ever know about yarn overs! http://explaiknit.typepad.com/let_me_explaiknit/2005/11/so_um_whats_thi.html