It's first, because it happens to be my favorite, and the site I participate in the most. Users share images of all sorts. There are many different categories like, art, funny, and cute (where I post an insane amount of pictures of my adorable dogs) that users post pictures in, hoping to get up-votes, or comments. The posts can range from hilarious and informative, to sad, confession type posts. It is an outlet for people to express themselves in any way they want. The images very often reflect the perspective of people in different areas of the world, regarding current events. For example, people from Australia commented on the conflict in Ferguson Missouri. Imgur is a great reference if you need a pick-me-up, or a reality check. A lot of people confess to using it as an anti-depressant, or an outlet for their creativity when they are afraid of criticism. The comments tend to be MUCH more friendly than other media sites. If you have ever read the comments on Youtube, you start to lose faith in humanity. Imgur comments are not like Youtube. Overall, it's a great community. If you're looking to understand what the majority of the youth is talking about, check out this site! Watch you though, it's addicting!
I am only going to talk about one other site. This is because, this site will show you all the other sites related to it, and lead you to the others on your own.