Time Machine to a disk connected to your Mac
There are other apps that can back up your files to an external hard disk, but Time Machine is simple, built-in, sure to be supported, and offers file versioning as well as simple backup; that is, older copies of your files are stored alongside the current one so that you don’t have to restore the most recent version, but can step back through time to grab older drafts. For this reason it’s a good idea to buy a hard disk that’s two or more times the size of your internal drive—so you have space to store lots of versions. (It’s all handled automatically for you.) Hard disks are cheap—a little over a hundred bucks for a 4TB drive, at the time of writing—but if you have a laptop with an SD card slot you could also consider fitting an SD card (even a Micro SD card inside a Nifty MiniDrive so that it’s nice and flush) to use as your Time Machine drive for extra convenience.
And certainly, if you’re using Time Machine on a laptop with a physically connected disk, consider those smaller disks based on 2.5-inch mechanisms rather than 3.5-inch desktop drives; they’re usually more expensive per gigabyte, but they’re “bus-powered,” drawing the power they need through the USB port rather than requiring a separate power supply, all of which means you’re more likely to plug the disk in and actually back up.
Good because: Set-it-and-forget-it easy, cheap, built-in, likely to be supported for a long time. Takes a snapshot of your files every hour, and makes it easy to retrieve deleted and overwritten files with its versioning feature. Can also be used to restore an entire system.
But be aware that: You need to have the disk connected for back ups to happen (fine on a desktop, but not guaranteed with a laptop), it’s slow to restore from if you replace a failed internal hard disk (you can’t boot from it), and it offers no protection against theft or local disasters such as fire.