A tumblelog is a quick and dirty stream of consciousness, a bit like a remaindered links style linklog but with more than just links. They remind me of an older style of blogging, back when people did sites by hand, before Movable Type made post titles all but mandatory, blog entries turned into short magazine articles, and posts belonged to a conversation distributed throughout the entire blogosphere.
Source: Tumblelogs (kottke.org)
I'd never run across the term "tumblelog" until tonight. This is what I've been wanting to do here for ages with asides, what I've done occasionally in OPML, have considered doing with sabifoo, and have done a bit from the bus via SMS on Vox.