Time magazine focuses on the web phenomenon, icanhasacheezburger.com:
Take a picture of a cat doing something cute. Then make up a caption–something witty that the cat would be saying if cats could talk. Bear in mind that cats can’t spell all that well and that they’re not so hot on subject-verb agreement either. Photoshop the caption onto the image, and post your creation on a blog. What you get is lolcats: lol for laugh out loud, cats for cats.
It’s easier to show lolcats than to explain it. The oldest known example–which probably dates to 2006–is an image of a chubby gray kitty looking at the camera and asking plaintively, I CAN HAS CHEEZBURGER? Later came a shot of a kitten in a state of feline outrage, standing over a plate of what look like clementines and meowing DO NOT WANT. A ginger cat caught in midleap, hind legs pedaling furiously, appears over the words INVISIBLE BIKE. A fierce-looking tabby crouches in a well-stocked refrigerator: IM IN UR FRIDGE EATIN UR FOODZ. You get the idea.
Here’s the one that started the whole thing:
Mental Floss has more on the origins of the lolcats phenomenon.
Anyway, it’s a very cute site.
It’s even spawned a few spoofs, like LOL Trek: