I recently read a blog post about what microblogging is. It mentioned Twitter and Jaiku. From what I’ve read, those belong to one type of microblog. There’s at least one other type, the tumblelog. A tumblelog is sort of a scrapbook, where ‘ordinary’ blogs are diaries/journals or even a sort of homepage (which in itself can be a lot of things, for instance a sort of business card).
I have a tumblelog on Tumblr, which I use mainly as a photoblog, but where I also post links and other shorter posts that I wouldn’t put on a ‘traditional’ blog. There are also others of roughly the same type, like Publr and Plurk (which is also a kind of social network, but then so are all to some extent). There’s also something called Soup, which is more or less a similar thing.