I just spotted this post on the Huffington Post and it explains polling better than anything else I have read....
By Natilie Jackson, Huffington Post:
Right on cue, the post-Labor Day polling bonanza has begun. For those who are tuning in to the presidential election for the first time, welcome! This is when we start getting tons of poll releases every day, many of which will show different numbers.
To read the article, click on this link.