There are couple different ways to interpret the word "backup." The most common form of backup is to choose certain files, and have a third party vendor store them in digital, retrievable copy for you. Generally, you store files in a specific folder on your computer, and your vendor syncs that folder to a folder on a server in some other part of the world. This is generally very inexpensive, and sometimes free or included as a part of a suite of services from a vendor.
This is a good option for client files. Even if you have a full system backup in place, having an organized place for just client files gives you, and your clients, some additional, easy security.
Good vendors are Dropbox for Business,
Box.com, and Sharefile.