Yup, bafalken is right, but also, anyone can rate the answer.
On how good it is (comprehensive, lucid, and generally how well it answers the question). Anyone can rate it on a scale of 1-5, which helps create an average rating and determines where the answer lies on the page (higher answers go up top).
Hope that helps,
Dara (aka editor)
The rating is established by the person of whom asked the Q:
based on the answer provided by the latter member, of which on a scale per relevance and as how well an answer is.