Key combinations for booting your Intel-based Mac.
When you are having trouble with your Mac, there are some key combinations you might want to know about in order to help you do some of your own troubleshooting, especially if your system is outside it's AppleCare or base Apple warranty. There's a great reference on the Apple website that I discovered while listening to Adam Christianson's Maccast podcast. (His details can be found in the December 29, 2015 show notes, under the OS X Startup modes and keys section.)
Some of these modes are really for more advanced users, such as Single User Mode or Verbose Mode, however there are several handy modes that the average user may find useful: Safe Mode, Recovery Mode or the testing modes for diagnostics - Apple Hardware Test (post June 2013 Macs) or Apple Diagnostics (pre-June 2013).
Apple Support Article: Startup key combinations for Mac.