Delta copying is a fast way of copying a file that is partially changed, when a previous version is present on the destination location. With delta copying, only the changed part of a file is copied. It is usually used in backup

Backup

In information technology, a backup or the process of backing up is making copies of data which may be used to restore the original after a data loss event. The verb form is back up in two words, whereas the noun is backup....

 or file copying

File copying

In the realm of computer file management, file copying is the creation of a new file which has the same content as an existing file.All computer operating systems include file copying provisions in the user interface, like the command, "cp" in Unix and "copy" in MS-DOS; operating systems with a...

 software, often to save bandwidth

Bandwidth (computing)

In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...

when copying between computers over a private network or the internet.