Tumblr is a decent site. I've used it. Now they have changed the interface of one's profile page, which looks decent. And yes as someone above said, it is a kind of blogging site with some other good features as well.
I forgot to mention Tumblr's best feature, the "reblogging" feature. When you find a picture, or a journal entry, or a video that you like on someone else's Tumblr, you can "reblog" it so that it appears on your own tumblr page. Then, all of your followers can see what you reblogged, and they can choose to "like" or reblog that post too. So, you see, these journal posts and images can spread very fast to a lot of people throughout tumblr, as the posts gain more "notes" (which are like 's) and become popular. If you are an artist, your artwork can gain viewers quickly because of people reblogging it to their own tumblrs, so that those people's friends can see it, so that they might reblog it too. Eventually, if your stuff is good, it gains hundreds, even thousands of "likes" and reblogs.
I guess Tumblr is best described as a blogging site where you can publish journal entries, pictures, music, and videos. Other tumblr users can "favorite" these posts. When you follow other people on tumblr, you can see their posts on your "dashboard", which is very similar to Facebook's "newsfeed". Then of course, you can search certain keywords or "tags" to find people who post the same things you like, and make friends that way.
At the end of the day, its just another blogging/social networking site. The format lends itself nicely to image uploads, which is why a lot of artists like to use it to post their creations.