What’s a tumblelog?
Definition : If blogs are journals, tumblelogs are scrapbooks.
You can also look at tumblelogs as slightly more structured blogs that make it easier, faster, and more fun to post and share stuff you find or create.
You can find more information on Wikipedia.
Tumblr is your friendly, free, and terrifically easy tool for creating tumblelogs.
More about Tumblr : http://www.tumblr.com/faqs