Using Ubuntu Linux/Introduction for Mac users
macOS's core, Darwin, is Unix-like. A lot of the commands you might use on the terminal, like
ls, etc. will work in Ubuntu because they both use Bash as their shell.
Intel Macs from all generations should be supported. PowerPC is less certain, since official releases are unsupported and drivers may not be available for all the hardware.
If you're using a hackintosh, make sure any EFI emulator or kernel hack you have set in place for macOS doesn't apply system-wide so that it doesn't collide with GRUB or its settings. If you plan on switching completely, you won't need to worry about any of it because Ubuntu will replace everything upon installation.