This has a lot of graphs in it so rather than posting it I will just put in a link and one paragraph.

"Now, I want to be clear: I am not making any claims about which pollster has the better sample of the electorate. I'm not singling anybody out for being right or wrong because frankly I do not know. I'm just pointing out that there seems to be disagreements among them that cannot be explained by random variation."

RealClearPolitics - HorseRaceBlog - A Note on the Polls