What are the different backup solutions?

Where simple backups don't work is In the case of a complete system failure, where you hard drive and operating system has crashed and needs to be rebuild.

I save to the cloud, isn't that enough?

Sync services like Microsoft OneDrive, Google Drive, iCloud and Dropbox are not backup solutions. Their purpose is to sync files and folders across devices so you can easily access them and are not replacements to backups.

Due to the nature of this syncing process all changes made are reflected across all those devices connected to that account. This means when a file is deleted, accidently or not that file is removed across all devices.

What is a 3-2-1 Backup Stragegy all about?

A 3-2-1 strategy means having multiple copies or your backup data - 3 in fact.
Two copies are stored locally but on different drives or media types. The last copy is stored offsite.