Tumblr is a blogging site. It's like a forum, but only one person posts to it, and posts are ordered from newest to oldest.
Since it's a networking site, you can follow other blogs. On your dashboard, which is pretty much the home page of tumblr, for every blog you follow, there will be a list of posts from every blog you follow, also ordered from newest to oldest.
It's more casual and more personal than deviantART. Use it if you want people to basically have a live news feed of what you do, and I don't recommend use it to archive things into a gallery like you would with dA or PJ.
Fair warning though, Tumblr works weirdly with pixel art, particularly small resolution GIF's and anything with a bpp of 8 or lower. Partially due to scaling issues a part of web design to keep posted images consistently sized.