It is a way of posting information to the web
you are limited to 140 characters. People write about what they are doing (usually boring), post links, all sorts.
I actually find it really useful (zimzum42 by the way!)
If you are smart with who you follow (whose 'tweets' you receive updates of) then you can get some really useful information out of it. Has been quite interesting following Lance during the giro, and there are loads of people who tweet about stuff I'm interested in and supply me with good links. Much quicker than checking all their individual blogs...
Yeah, there's a load of dross out there, but if you can cut through that there are some real gems in there too, it's like a microcosm of the web...