Early in the history of the web, folks who maintained sites often kept a "log" of changes they made to the "web." When blogging software emerged, humble web logs got lost in the excitement of social journaling.
Blogging software was a breakthrough for regular folks editing the web but that was years ago. Nowadays there are many ways we all edit the web—Blogger, Flickr, Del.icio.us, and YouTube are just some of the ways we put new stuff up every day. Often, we use more than one of these types of services.
Now everybody is a web master. So how do we pull all this content together into one place to show a log of stuff we have recently added to the web? Tumblr is the modern web log—it's all your recent updates to the web. Also, it does a great job of displaying the amount of content you are really contributing.
For example, my Tumblr page collects posts from two blogs, all my twitters, my del.icio.us links, and anything I add to Flickr. It's all made possible thanks to the machine readable web—a vast collection of structured data versions of all our thoughts, home videos, and funny pictures of cats.