Dropbox is a file sync and sharing tool. I’ve been using it for the last 7 months and it’s saved my rear end a few times already.
How it works is you install the client and it creates a directory in your Documents folder (for Windows, the location is different for OSX and Linux) which is synchronized to the dropbox servers. You can then install the client on another computer and anything in your “dropbox” that has been synchronized will be downloaded. Any changes you make on either machine will be synchronized with each other as well as the dropbox server. You can access your files by installing the client or logging into the dropbox server via browser and downloading files from the website.