Basically it's a series of questions (six pages) that finds where you lie in normal political left-right (economic) and the 'up-down' of the social aspect (authoritarian or libertarian).
As with most of these tests then there's some questions where you think "well I disagree with that, but I don't exactly agree with the exact inverse, so what is it going to do to my results?" but my final ratings were:
Your political compass
Economic Left/Right: -3.50
Social Libertarian/Authoritarian: -3.44
[edit] Just re-took it to see how it affected things (still trying to be truthful) and came out with:
Economic Left/Right: -4.88
Social Libertarian/Authoritarian: -3.33
So obviously my Libertarian value is about correct, but my classic left-right rating is a bit undecided depending on how I think about the questions.