A snapshot represents a frozen image of a volume. The source of a snapshot is called an "original." When a snapshot is created, it looks exactly like the original at that point in time. As changes are made to the original, the snapshot remains the same and looks exactly like the original at the time the snapshot was created.
The snapshot feature is very useful for performing data backups. In order to perform a consistent backup, the volume that is being backed up should not change while the backup is running. This often means the volume must be taken offline during the backup, which can be a significant inconvenience to users. With snapshots, the volume can be kept online. A snapshot of the volume is created and the backup is taken from the snapshot, while the original remains in active use.