Duncan Ellis just did a blog post called, "Backups," so I thought I'd do one also. Always a great topic. As the techies say, if your data isn't in at least three places, it doesn't exist.
Absolute top priority is my own stuff, stuff created by me. Which is words, almost entirely.
Different stages of writing have different systems.
1) Drafting and notes. These days, early drafts and notes are done in ColorNote (for Android – I believe it's similar to Evernote). So, it's backed up to "the cloud," and also across multiple devices.
2) Story installments. When the next part of a story is starting to shape up, I copy it from ColorNote and into a new WordPress page. This is where I do serious editing (as a draft, before it goes live), including seeing how it fits with what has come before.
Part of the latter process involves a PHP script I wrote which pulls out the content of a series of WP posts or pages from the database and assembles it into one HTML file. These HTML files (each named with the date) are all saved in different locations, and also emailed to my Kindle so I can hear them read out loud to me. So, they're backed up on different devices and drives, in my Sent email, and also on the Web (on the blog itself – and I get weekly backups of the blog database).
3) Finished works. Backed up as in #2, but also in Google Drive, and in folders on my websites (these backup folders are outside the "public_html" folder and are therefore not accessible by Web browser).
I'm not so protective of my blog posts. Weekly database backups are good enough for that.
Stuff that I wouldn't like lose. Mostly music files. They're saved across various hard drives, in a "system" which, I confess, prioritizes redundancy over organization.
They'd be hard to lose, but they're also often hard to find. 🙂
I don't do state backups (backups of an entire computer at one time). I do most of my work in Android these days, and it's pretty easy to set up a new tablet or phone with my usual apps and so on. My actual computers run Puppy Linux, which is a LiveCD distro, meaning it boots from a CD which holds the operating system and the programs. So, unless the CD breaks (and I have spares), I'm okay.
So, what's your priority for backing up?
(You do back up, don't you?)