Basically, for general usage it can be like tinypic or imageshack, but you can just keep the files on a folder on your computer and it automatically uploads it to the web, and you can right click pictures right on your computer and get a link to it so people can view or download it. More or less, it just makes sharing stuff a lot easier...
It can also sync your files across multiple computers and you can make specific folders that will sync between you and a friend (or multiple friends). Oh and it doesn't just work for pictures either, you can do any type of file.
I was actually in on the invite only beta, ages ago... But, it seems to have gotten a bit more popular now.
Anywho, if you haven't signed up and want to, here's my referral link:
https://www.dropbox.com/referrals/NTM3NDU5OQThe advantage of signing up via my link as opposed to just signing up without it is that if you use my link we'll both get an extra 250MB of storage space (default free storage space you get is 2GB), so it's a pretty good deal. Besides, it couldn't hurt me to get some more storage space I figure...
Anywho, it's pretty neat. That video they have up on the referral sign up page is pretty lame... the one they had back in beta was a lot better. But yeah, if you didn't know about it... now you do.