Labour - technically left wing, socialist party, strongly linked to the unions. Under Blair they have moved away to a more Democrat "Third Way" stance.
Liberal Democrats - Social liberals. Like Libertarians on social issues, and lefties on economic ones. Powerful at the local level.
Conservatives - right wing, party of the landowners, very rich and clergy. Generally divided into a libertarian/constitutionalist wing (small) and traditionalist wing (large).
Green Party - left wing environmental group, not very popular
UK Independence Party - right wing, xenophobic, libertarian economics, opposed to the European Union and immigration, probably not racist though.
RESPECT- left wing socialist environmentalist group, main member being George Galloway MP, friend of Saddam Hussein.
And of course, the BNP. Those are the largest parties.