Found this in the Support Center. I assume this is still valid for the time being.

Backups vs. Snapshots
DigitalOcean users have the option of backup and snapshot features. The main difference between the two is that snapshots can be generated manually, while backups are run automatically weekly.

Additionally, the server shuts down during snapshots, but stays powered on during backups, which happen in the background.

Once the files are saved, both the snapshots and backups are stored on hardware separate from the droplet resources.