[RobertGraham]

Backup is for the configuration files, snapshot is a complete image of the whole platform.

Here's the description of snapshot from the PDF:

Commands to take a snapshot of the entire system and to restore the system, from
the snapshot, are available. The system can be restored at any time, and at boot
time the user is given the option of booting from any of the available snapshots.
This feature greatly reduces the risks of configuration changes.
The snapshot and revert commands can use an TFTP server or a SCP Server to
store snapshots. Alternatively, snapshots can be stored locally.

Here's the description of backup from the same PDF:

Backup the system configuration. You can also copy backup files to a number of
scp and tftp servers for improved robustness of backup. The backup command, run
by itself, without any additional flags, will use default backup settings and will
perform a local backup.