basically you install a service and designate a folder as your "dropbox".. you can do this on as many PC's as you want. When you add content to the folder, all your registered machines with it installed synch the folder together.
So I have one common "code" folder on my 2 home laptops and work PC in synch. It also stores the data in a secure web repository and has version control.
It is useful for synching multiple machines or collaborating with a group of people that have synched dropboxes to the same account.
It works over the web on port 80 so you don't have to worry about VPN's or corporate firewalls or anything.
it also works on almost all platforms and is free.
disclosure: im not affiliated with dropbox, just a happy user.