A Sober and Smart Analysis of Polling Data
The heat of the campaign has put pollsters in the spotlight, and this article does the best job I could find of explaining the current controversy, polling in general, and polling data on this election.  538 is a great site to use to track election polling.  Another is Real Clear Politics.

538 finds that the current criticism "are largely unsound, especially when couched in terms like “oversampling,” which implies that pollsters are deliberately rigging their samples."  Then 538 examines if there is a history of bias in election polling and concludes that there is not.

"The polls have no such history of partisan bias, at least not on a consistent basis. There have been years, like 1980 and 1994, when the polls did underestimate the standing of Republicans. But there have been others, like 2000 and 2006, when they underestimated the standing of Democrats."

And our recent experience with polling data indicates volatility may be more salient than bias.