A backup file is created (often automatically) to hold a copy of data which can then be restored at a later date should the need arise. It is still relatively common for data to become corrupt or accidentally deleted so having archives of any important data should be an essential part of any disaster recovery plans.
There are generally three types of backup:
Full- Each backup file contains an exact copy of the original data
Incremental - A backup of all data that has changed since the last backup was made.
Differential - A backup of all data that has changed since the last full backup was made.