I must admit, I was a little leery about Dropbox when I was first introduced to the online storage solution by a fellow writer, but it's come in handy and has simplified my life.
Here's how it works. Dropbox stores synchronized files in a cloud so they're available on any device I've installed this tool. So if I save a file to Dropbox, I can access that file via my smartphone, iPad, laptop, etc. And if I make a change to this file on any of these devices it is automatically synchronized on all of them.
So if I'm waiting in the doctor's office (and waiting and waiting and waiting) I can use that time wisely by working on my story by accessing it on my smartphone. I can even invite my agent to a folder and she can access the same story.
Here's a video that explains how it works much better than I can in writing and an article from PC Magazine. Do you use Dropbox?