Snapshots are on-demand, instant copies of your servers disks. You can create a snapshot anytime. You can restore to that snapshot anytime from your client area to roll back to the server state at that particular time.
We recommend that you power down your server before taking a snapshot to ensure data consistency.
You will need to buy snapshots while ordering process. Here is a use case.
Let's say you bought 100 GB of snapshots. This enables you to take snapshots upto 100GB in total against all snapshots you have.
Also, if you have bought a server with 80GB of disk space, the snapshot will not necessarily be 80GB. Snapshots are taken in a smarter way and it only counts storage which you are actually using. For example, a snapshot for a server with 80GB of disk space may be less than 20GB. In this case 20GB will be counted against your Snapshot quota.
You can always delete snapshots that you don't need, it will free up your Snapshot quota.