The site is essentially a bulletin board with entries posted by registered users. All entries are listed in categories of interest called reddits that redditors can then vote for or against. These are then displayed on the front page. Comments can also be posted up and those voted for or against too. It is important to understand who the users are on Reddit and how to post before venturing in.
The infographic below gives insight to this powerful social news platform!
The Reddit Marketing Field Guide [Infographic]