What is the difference between Evangelical and Protestant?

Just the other day, I heard somebody talking politics and contrasted the Roman Catholic church's political influence with "the Evangelical vote". In this sense, as is often done in US media, the word seem to be used as a synonym of 'Protestant'. However I get the feeling there is another layer to this. Where did the term 'Evangelical' come from and how is it distinct from 'Protestant'. Used properly, what group(s) is it generally understood to include or what world view does it describe?