'lol' means 'laughing out loud', as other people have mentioned. It's not just a 4chan thing, all Americans on the internet are familiar with the term. But as mentioned above, it doesn't always mean that you *are* laughing out loud, just that you're amused at something.
Other variatons on it include:
ROFL: Rolling On the Floor Laughing
LMAO: Laughing My Ass Off
ROFLMAO: Rolling On the Floor Laughing My Ass Off
lulz: Same as LOL, but a more 4chan/Anonymous-specific version. Also used as a noun (example: "I posted scat porn on this kid's forum for the lulz.")
Someone else here can correct me if I'm wrong, but I think 'wwwwwwwwwwww' is the Japanese equivalent, or close to it.