Polls, polling. Here is an explanation of polling from electoral-vote.com a site that is kept by a Kerry supporter who sets out his methods so that one can judge if the site is presenting material fairly. This discussion of how polls are conducted and the intricacies of sampling is vital to understanding how poll results can be skewed or presented in biased ways.

And, from Gallup's own website, their FAQ on surveys and polls. Frequently Asked Questions