Tumblr is a blogging service that was founded in 2007, and was acquired by Yahoo in 2013. Tumblr is currently the largest blogging site on the web, although it has also redefined what exactly "blogging" is.
Unlike almost every other blogging platform, Tumblr does not have a commenting feature. The closest it has is the ability to repost someone else's post, with added commentary. Tumblr also, compared to blogging sites of the past, is not primarily text-oriented. Much of the content is pictures, and animated gifs. There is also a large section of pornography blogs in Tumblr, although that seems to fall outside of what could be considered the cultural nexus of Tumblr.
With Tumblr, function follows form: the visual nature of the site, together with the inability to use it for discussion, means it is a site more for sharing images, funny reposts, and instantaneous feelings, rather than for deep interpersonal or political discussions. There is a lot of social discussion on Tumblr, but it is more along the lines of "This popular movie was sexist and we should be mad!" than nuanced debate. Tumblr is also a center of fandom, especially with Doctor Who, Sherlock, Homestuck, and Supernatural.
Tumblr's format, and its large user base, make it a large source of creative and interesting content. Creating a tumblr and sharing information on there is so easy, that it is one of the internet's main center for the manufacture and spread of memes. However, with Sturgeon's Law in mind, Tumblr is also home to much material that is repetitive, stale, or derivative. My best explanation of Tumblr is that right now, it is the most internet part of the internet, where the internet is at its most creative and crappy. How long Tumblr maintains this role is an open question, but given past internet trends, it will probably last a year or two before being eclipsed by something else.