Dropbox - Online backup, file sync, and sharing made easy.

Various of my friends have invited me to join Dropbox - Online backup, file sync, and sharing made easy..

Unfortunately, when I go to the site, I'm, invited to "watch a video".

I don't want to "watch a video. I never watch videos on my computer because:

1. They hog bandwidth
2. They are jerky and incomprehensible and annoying

So can anyone tell me in plain language what Dropbox does, and why I should join it?

I've already received a notification from my ISP that I've used 80% of my monthly bandwidth (and the month isn't half gone yet), so I'm not going to watch a video when I know that I'll have to hear a second's incomprehensible sound, and wait ten seconds for the next bit to load and then watch another second. And find I've wasted all my bandwidth on such junk.