From a Beyond the Fringe sketch by Peter Cook1, here's an explanation (Pre-Blair) of American Politics for the British:

'There's the Republicans, who are like the Conservative party, and there's the Democrats, who are like the Conservative party.'

Of course, nowadays, it's more like:

'There's the Democrats, who are like both UK parties, and there's the Republicans, who are like nothing any sane British voter* would ever vote for.'

*Shome mishtake, shurely?

1: Thanks, Stealth Munchkin!