I use
MEGA to back up my documents. This synchronises the folder on my computer with a folder in the cloud. It's free for up to 20 GBytes. If I set it up on another computer, it can synchronise the folder on the second computer with the cloud version and hence the first computer. It works with Mac, Windows and Linux.
It means that I can have two local copies and one off-site copy.
The company is now owned and overseen by the New Zealand Government, so is pretty trustworthy.
I don't usually backup the whole computer, as I don't think it is worth it. It generally takes longer to recover the whole computer, after having worked out which backup to restore, than it is to just re-install Windows with fresh installation media. That gives me a fresh, clean installation. Then I just restore my documents.
Archived documents, such as photographs, scans, all the letters you sent to your kid's primary school, are all backed up locally on two separate drives. They get re-written to new drives every 5 years or so.