I spend most of my time on Tumblr, and it's a truly wild place. There's so many different types of people there that you can't really put a solid description on it, but basically it's inhabited full of a shitton of crazy people who think everyone else is crazy.
Tumblr for me is fandoms: snk, naruto, lok, etc. It's a nice way to find cool art / text posts / RPs etc. Tumblr is like Facebook for me, except it's less personal (let's face it I don't give a fuck about 90% of my friends on facebook) and the people I follow are generally friends or nice fandom blogs that I can talk to.
Uhhh... don't repost and don't follow hipster blogs (dear god no).
Fandoms are a huge part of Tumblr, and it's probably the most diverse part. You've got your Sherlockians, Whovians, Supernaturalists, Potterheads, Bronies, Otakus, Gif blogs, Gif makers, Fandom-specific blogs, Multifandom blogs, Anime blogs, Anime gif blogs, and so on. The fandoms are full of every type of person imaginable, from the crazy rabid fangirls to everything else. Fandoms aren't the only part of Tumblr though, there's band blogs, porn blogs, yaoi blogs, specifically gay yaoi blogs, photography blogs, art blogs, relatable blogs, and hipster blogs. Everyone basically hates each other to some extent for some reason, and as demonstrated above a lot of that hate is directed towards hipster blogs. There are many different types of hipster blogs, more than most people care for really, and honestly I don't give enough shits about hipsters to list the different types.
Tumblr can be both a beautiful and scary place, mainly due to all the hate people love sending to each other. People like to romanticize insanity for some reason and therefor like to pretend to be insane, mainly by making a lot of questionable comments on posts. Thus the saying goes, "This site is full of psychopaths."
Another reason why Tumblr is scary is because of the society that it runs, where everything is literally some form of stuck-up sass, where it’s taboo to care about other people or activities because if thay can’t do it or if thay can’t socialize then you shouldn’t do it ever, where people think it’s cool to say shit like “i’m going to punch every cheery person in the throat” because they’ve become so self-absorbed they become absolutely miserable, and if they're miserable then everyone else should be miserable.
Anyways that's just the tip of the iceberg of which I've described, and despite what I've said Tumblr is a pretty fun place one you get used to it.