iMore:

Sometimes your Mac isn’t running properly, so you need to reset the NVRAM and PRAM. Sometimes you need to boot in Recovery Mode in order to reinstall an OS or recover your hard drive via the internet. Whatever your needs, you can boot your Mac in a variety of modes in order to complete a task before startup; all you need to do is press some keys down and wait for the chime (unless you have a later MacBook Pro, in which case I’m sorry).

Here are all the keyboard shortcuts you can use when starting up your Mac!

I had a kernel panic situation a couple of weeks ago and was glad I had a cheat sheet of these lying around.