reddit is a user-driven link aggregation site. Registered users ("redditors") can submit links and rate other links by voting them up or down. When submitted, a link starts out with one point. One point is added to its score for every user that votes it up, and one point is removed for every vote down.

If a link has a high enough score, it will move to the front page, where more people will notice it and either vote it even higher, or vote it down, forcing it back into obscurity. The most popular links will rise to the top of the front page, where they're practically guaranteed to stay for several hours, and will receive even more upvotes.

The score of a submitted link effects the score (karma) of its submitter. All users start out with 1 point of karma; some have earned over 100,000. The exact algorithm for calculating karma is not known, but its effects are. As a submitted link's score increases, its submitter's karma will increase, and if a link is downvoted, its submitter's karma will decrease as well.

Karma has no noticeable effect on anything, it simply serves as a way to rank users and rate how much the reddit-using community enjoys their submissions. By submitting links that almost all of the reddit community finds appealing, significant karma can be gained easily.

Hot topics

Links regarding these subjects have the best chance of being rated favorably:

Writing headlines

Most redditors will upvote or downvote a link based on its title and whether or not they agree with its implications, sometimes without even clicking it, so give it some thought. A brief summary of the link's content is always appropriate, and including a quote from the link tends to work well. A vague but interesting snippet will make people curious and willing to click the link, and probably the upvote button as well.

Titles in the format of "The best [anything] ever" or "The worst [anything] ever" will often make it to the front page, as will "incredible", "amazing" or "unbelievable" photos and videos (be sure to add "[video]" or "[pic]"). "Upvote if you [something about reddit]" also seems to be popular, as does any title starting with "Why" or "What" that isn't a question.

Miscellaneous

Use the reddit bookmarklets for easier submitting of any page you happen to find. Links seem to rarely go below 0 points, so if you see something worthwhile, submit it. There's really nothing to lose. That's how reddit is supposed to work, anyway.

If one of your links is immediately voted down to 0, there's pretty much no chance that it will ever become popular. I guess some people just can't resist downvoting new links, though.

While each link's comments can be voted up or down, this seems to have no effect on the commenter's karma, so you can go ahead and say whatever stupid, contrarian, inflammatory shit you want. I do. This guy does.