Holy Moly Barrold Bonds,
What a sweeping generalisation!
Reddit is simply a forum where people post links or texts on subjects that interest them. They post them to 'sub-reddits' which are generally set up around specific topics. Some of the more popular sub-reddits are Pics, Funny, GIFs, News, World News, etc. Other users then upvote or downvote your post depending on whether they liked it or not. If you have a favourite TV show, or a niche interest, I can almost guarantee there is a sub-reddit for it.
Of course as people can set up sub-reddits for whatever they want, there are some that you would avoid. Just like 4chan - a great site for pictures and info, but if you don't want to have to pour bleach in your eyes, avoid the board called Random.
Anyway, back on topic....