Both are paid add-ons - choose them when ordering the server, or add them later through a ticket.
A snapshot records the server's entire disk at one moment. Take one before risky changes: a system upgrade, a configuration change, installing new software.
How many snapshots you can keep depends on the option you bought. A snapshot stays until you delete it.
Backups are made automatically every day and the last 7 are kept; the oldest is replaced by the newest. Back up now makes one by hand.
Restore on a backup replaces the server's entire contents with the backup from the chosen day. It asks for the server's name, because it cannot be undone.
Snapshots and backups are stored off the server's disk, so they also protect against a system failure. For your most important data (e.g. databases) it is worth keeping a copy of your own as well.