It's really funny to see people from one site waging war against people from another site. In this case it's 4chan versus Tumblr.
4chan is an online message board which has almost any topic you can think of. From looking for anime torrents to pictures of people indecently exposing themselves, they have it all on the boards of 4chan.
On the other side of the battlefield, Tumblr is a blog hosting site with a lil Facebook (or Twitter) flavour thrown into the mix. This is because every blog post on Tumblr can be shared easily and be re-posted on one's blog with just a click of a button.
So why the war? 4chan claims that Tumblr has been stealing ideas and memes from them without giving them credit.