We need backup to protect against data loss from hardware failures, viruses, accidental or intentional data destruction, theft, natural disasters, or computer inaccessibility. The three main backup methods are full backups that copy entire data sets, incremental backups that copy data changed since the last backup, and differential backups that copy data changed since the last full backup. Backup data can be stored on local hard disks, optical discs, tapes, solid state storage, or remotely using online backup services, with an optimal backup strategy incorporating onsite, offline, and offsite data protection.