There are a few kinds of cloud backup services, depending on how you want to back up your digital data.
System Data Backup - This approach backs up ALL the files in your computer system, not just your digital data. It is designed for individuals who are not computer savvy and are mainly concerned about being able to access some file(s) in the event they are unable to use their computer. Two such services are CrashPlan and Backblaze.
File Sync Backup - This approach creates a single place on your computer where you can place your digital data into; and it will automatically sync up with the cloud service, so that no matter where you are or what kind of computing device you have, you will have access to your digital data stored with the service. Two such services are Dropbox and OneDrive.
Cloud Storage Backup - This DIY approach allows you full control on what gets stored in the cloud as backup. It is similar to using a USB flash drive or an external hard drive. Two such services are SpiderOak and Tresorit.
Hybrid Cloud Storage - This approach is a combination of the above three approaches. This avenue is great if your needs require more flexibility while keeping things as simple as possible. Two examples of such services are Bitcasa Personal Drive and CrashPlan
DIY Cloud Storage - This is completely DIY approach, for those who are very technically savvy and are concerned about their privacy on the cloud. There is a comprehensive article on how to set this up.
The Cloud has provided a practical and relatively painless way to have backups that are available in case your place becomes unavailable in some fashion (burned down, natural disasters, robbed, electrical outage, traveling) and having considerations like multiple versionf of your digital data. I consider the cloud to be a viable secondary method in a backup strategy.