On that site one has 7 buttons to rate a message: 1, 2, 3, '+', '-', '+1' and ':)'
1, 2 and 3 affect score (=karma) of the message and its author based on the level of the person who rates it (i.e. 2 from a 5th level person would add 5*2 = 10 etc)
'+' and '-' show that you agree/disagree with that comment, but do not affect the score.
'+1' adds 1 point to score independent of rater's level.
':)' shows that you find this comment funny.
I translated rules from this page http://rsdn.ru/forum/Info.aspx?name=info.forum.rating and as you may see, there are more rules that help to prevent excessive you-rate-me-I-rate-you behaviour, keep score up-to-date (your level depends only slightly on your all-times score but greatly depends on the marks that you've got during the last month) etc.
If people are interested, I can translate the rest of the rules.