The Polls

Does anyone know a lot about polling? How do you know which is more correct? Here are some examples.

Reuters/C-SPAN/Zogby Poll - Obama +9.6

AP presidential poll - Obama +1

Real Clear Politics Poll - Obama +6.8 (Average for the polls)

If you look at the Real Clear Politics link it shows numbers from many polls of which non seem to be very close within each other, but perhaps the average of them gives us a good indication of what the polls really are. Anyone have an insight in polling?

UPDATE
Here is a link to an article from a retired pollster that seems to answer some of the questions I have had concerning the polls this year.
Polls show Obama +14 all the way down to +1