What is the difference between backup and redundancy?

They protect against different failures, and having one does not give you the other:

• Redundancy keeps you running - a second internet service, a spare laptop, a mirrored disk. It answers hardware and service failure.
• Backup lets you go back - it answers deletion, corruption, ransomware and mistakes.
• A mirrored disk is not a backup - it faithfully mirrors the deletion too.

Most small businesses need a little of both, and usually have neither tested.