Dropbox is a file hosting service operated by the American company Dropbox, Inc., headquartered in San Francisco, California, that offers cloud storage, file synchronization, personal cloud, and client software. Dropbox has software for pretty much any platform including Windows, Mac and Linux. Also there are apps for Android and iOS. There is a very useful web app as well.
With Dropbox you can sync your files to the web. Share files with others. With the new version you can even merge Google Apps and Microsoft Office for the Cloud apps.