If you haven't heard yet, the latest internet meme is LOLCats. It's been popular for several months now, which means the fad will probably die soon.
Basically, it consists of taking pictures of cats and putting grammatically flawed captions on them. Some of them are hysterical. Some of them are not safe for work.
Each Saturday, on Fark.Com, there is a message thread that primarily turns into a collection of these pictures, thus making it Caturday.
It takes awhile to load, but you can see last week's thread here:
You can find a list of recent cat threads here.