Anyone can upvote submissions and comments. Every upvote helps float the item higher, and counts for the submitter or commenter as a point of karma.

People with enough karma (currently 500 points) can also downvote (with a few exceptions). Downvotes are pretty much the opposite of upvotes, floating the item lower, and taking karma from the commenter.

The exceptions to downvotes are that you can't downvote:

* A submission.

* A reply to a comment you made

* A reply to a submission you made

* Something that's really old

Did you check out the FAQ before asking? There's a link to it at the bottom of (very nearly) every page: http://ycombinator.com/newsfaq.html

There it says:

    Why don't I see down arrows?

    There are no down arrows on submissions.
    They only appear on comments after users
    reach a certain karma threshold.

(although I see someone else has already quoted that)

No, I didn't! Didn't know of it, although I should have gone looking for one.

From the FAQ:

  Why don't I see down arrows?

  There are no down arrows on submissions. They only appear on comments after users reach a certain karma threshold.

I believe it's around 500 points.

Judging from the empirical evidence my own comments afford me, this ability is safeguarded for only the most short-sighted and humor-deprived HN users. (Bait.)

Note that humour does not travel well, sarcasm and earnestness are frequently confused with each other, and standards of who it is polite to be rude about vary.