Brand name cloud-based file storage system with live syncing to a web app, desktop apps, and mobile apps. Not to be confused with the native folder on OS X which people can "drop" files into.
Dropbox is a service by which documents can be backed up to the cloud, shared, and managed from any internet-enabled device. There are apps for OS X, Windows, Linux, iOS, Android, and Blackberry. There is also a web interface which can be accessed via a web browser on any computer.