Backup describes what they are for. There are many ways to do backups. The main thing is that backups are archive stored away from danger. There are different kinds of danger and different need for protection.

Snapshots can be backups depending on where they are stored usually not if stored locally. For example, RDS snapshot is backup for database going down but not account being deleted or region destroyed. Generally, snapshots are way to make backups to more durable medium.