A great tutorial can be found here. From the article:
In case you are wondering, Dropbox is an online storage site that allows you to back up and sync your files right from your desktop. It comes with a desktop application that you can install in your OS and backing up files is just a simple drag and drop to the Dropbox folder in your hard drive. Any files added/removed from the folder will be automatically sync with the online server as long as you are connected to the Internet.