A blogging platform emphasizing customizability and ease of use. Tumblr allows users to post text, images, video, links, quotes, and audio to their tumblelog, a short-form blog. Users are also able to "follow" other users and see their posts together on their dashboard as well as like or reblog other blogs on the site.
Features include: (1) Mobile posting (2) Mobile browsing (3) Tumblet widget (4) Post via Instant Message (5) Embed your tumblelog (6) Bookmarklet (7) Feeds (8) “Public” or Private” groups (9) Domains (10) Several customizable themes are available or create a custom HTML theme for your tumblelog. (11) Build your own apps with the Tumblr API.
Here's an article on it from BrandWeek: http://www.brandweek.com/bw/content_display/news-and-features/direct/e3ibdf529f18374f6c94a8e9a9a8af1b137