"upvote" => "to vote up"

"downvote" => "to vote down"

If you find something useful or helpful you can vote it up (give an "upvote"), e.g. with the up arrow to the left of this question/answer. And if you find something not useful or helpful you can vote it down.

In both cases (especially in the case of the downvote) a short comment or explanation would be appropiate and / or appreciated.

also see http://en.wiktionary.org/wiki/upvote