Reddit is a social bookmarking/link sharing site along the lines of Digg or StumbleUpon. More than this though, it's an aggregated content site where you can find content voted to the top by other users and thereby selected for its popularity among the community. It was founded by Steve Huffman and Alexis Ohanian who were roommates and students at the University of Virginia and subsequently acquired by Conde Nast Publications in 2006. Today it operates as a separate entity but with Advance (Conde Nast's parent company) as the largest shareholder.
Reddit's mission statement is to become the 'front page of the internet' and it achieves this by letting users select the sub communities (called Subreddits) that they want to subscribe to and then showing them the top voted content for that community each day. You can find these at reddit.com/r/(nameofsubreddit). As a rule, any content shared to Subreddits will need to gain a certain amount of traction before it starts appearing on the home page for particular users.
If you were to set up your own Reddit account then, you would be able to select the subjects and categories you were interested in and then Reddit would show you the top content from each of those categories every day. This would likely include blog posts and other external links, alongside 'self-posts' which are essentially just text posts that are designed to stimulate discussion within the community. You can also choose to view Subreddits on their own and that way only see the latest and most popular links and discussion for that given subject.
Subreddits and the front page alike have multiple separate categories accessible by tabs which include 'Hot', 'New', 'Rising', 'Controversial', 'Top', 'Gilded' and 'Promoted'. 'Hot' is the front page of that sub, while many visitors will also check 'New' and 'Rising'. 'Top' is comprised of the top voted posts of recent time in that sub based on popularity alone.
As an active user of Reddit you will be able to up-vote and down-vote any content yourself, thereby influencing which content gets seen and which doesn't. You'll also be able to comment on the posts of others and engage in active discussion. Each time one of your own posts or comments gets up or down voted this will then contribute to your overall 'karma' on the site. You have two separate karma values - one for content you've shared and one for comments you've posts. Other users will be able to click on your profile and immediately see your previous posts as well as your rating. This way your overall karma can end up positively or negatively influencing the respect given to your subsequent posts. Some Subreddits also have 'minimum karma' requirements for posting or even commenting which is a device intended to prevent spam.
Subreddits are ruled over by moderators, as with most forums, and they have the ability to delete posts, to add tags and to ban users. Anyone can create a new Subreddit and in doing so, you automatically become a moderator. At the same time you then have the option to add more moderators to help you manage the page.
Those are the basic marketing rules of Reddit. If it sounds complicated, then that's only because there is a lot going on under the surface. To use Reddit you need to go to Reddit.com and read what's there. On the other hand, to promote there all you need to do is to create an account and then click the 'submit' button. Understanding the nuances under the surface though will help you to get more from it, both as a consumer and as a marketer.