First thing is, are we talking about a laptop, a desktop, a server?
Unless you are certain you are the only user, it’s best to boot from a live-USB or a second partition while you back up your system. Even if you are certain you are the only user, there are some files you don’t want to copy if the system is running, and i don’t have a list close to hand. Things like /proc and /sys and /media that are dynamic and get constructed during bootup.
Now, linux is somewhat different from Windows in a very good way. If you shut your system down, it will clean up the stuff that needs cleaned up, and if your system is not running from the root partition, then the root partition is just like any other. Shut your system down, and you can use rsync (or even dd if you’re that patient) to make a copy of the disk.
Then, not only do you have a backup, you can copy it onto the drive of another system with similar hardware (important if you are using proprietary drivers) and boot it up. The only files i’ve run into that cause any problem are those with the “inviolate” attribute, and there are very few of those. The only one i recall is part of extlinux that has to stay put, but that’s not really an issue unless you’ve installed extlinux, and last i knew it was single-partition so it wouln’t handle multi-boot. Grub is easier, whether it’s “better” or not.
Your backup won’t be bootable, but other than that it’ll be an “exact” copy. I’m running a copy of debian-jessie at times, and i’ve chosen to freeze it due to continually unfixed problems i no longer care about. With MATE, i have a backup partition too. If i let synaptic or whatever update tool, apply all the updates, i always back up the system first, in case something untoward happens due to some update.
IOW it can be done, it’s not rocket-science, but if you’re new there are a lot of parameters to figure out here and there. There’s enough end-user configuration stuff with linux that the desire to keep what you’ve figured out is kinda min-spec-sanity imo, but mileage always varies.
