Dropbox is a file hosting service that offers cloud storage, file synchronization, personal cloud and client software.
The Dropbox software enables users to drop any file into a designated folder on the user’s computer or device. The file is then automatically uploaded to Dropbox’s cloud-based service and made available to any other of the user’s computers and devices that also have the Dropbox software installed. This synchronization keeps the file up-to-date on all systems. When a file in a user’s Dropbox folder is changed, Dropbox only uploads the pieces of the file that have been changed, whenever possible.

Source: https://en.wikipedia.org/wiki/Dropbox_(service)