iCloud will back up a few things on your computer—photos, contacts, calendars, etc. But, it iCloud is not a comprehensive backup solution for your whole computer. Here’s a list of what iCloud actually backs up.
You need to back up everything on your computer—not just a few bits of Apple data. There are several cloud backup solutions out there. My two favorites are CrashPlan and Backblaze. Both offer unlimited backup space for about $5/month. I have had some bad experiences with Carbonite, so I do not recommend using them.
Once you have a cloud backup in place, it’s also a good idea to have a local backup as well. Apple’s Time Machine is perfect for this task. Time Machine backs up all of your files to an external hard drive so that you can restore them later or see how they looked in the past. It’s best to use (or buy) an external hard drive that’s about 50% larger than your computer’s hard disk. That way, Time Machine can store more backups from the past.
Long story, short—iCloud is not a full backup solution. Use a different service to back up your data. It’s best to have both a local (i.e. Time Machine), and cloud backup.