There MAY BE A TIME WHEN YOU NEED TO TROUBLESHOOT A PROBLEM IN OSX, A NEED TO BOOT INTO A BACKUP DISK OR LAUNCH INTO SAFE MODE TO DIAGNOSE A STUBBORN APPLICATION. IT'S BETTER TO BE READY than GET READY SO KEEP THIS LIST OF STARTUP OPTIONS ON HAND FOR THE OFF CHANCE YOU MAY NEED IT.

Manually restart:

This method was intended as a last resort option when your computer is locked up and everything else fails. Hold down the power button on your Mac until it shuts off. Then release and then press the power button again to start your Mac. 

Prevent automatic login: 

For those who have automatic login enabled and need to temporarily disable it. During startup when you see the spinning gear progress indicator Press and Hold down the Shift key to access your login screen.

Disable login items and Finder windows from startup: 

This is a great one to have in your tool belt it allows you to open OSX with a minimal number of apps running. Hold down the Shift key after clicking the login button to boot into this start up mode.

Select a startup disk: 

Sometimes you might have more than one bootable drive in this case you may want the option to choose your startup disk. Press and hold the Option key during startup. If you need to open the optical drive when choosing a startup disk, press Command(⌘)-Period(.).

start up from a CD or DVD: 

Press the C key during startup will boot you right into any bootable CD in your drive.

Eject removable discs: 

Hold down the mouse or trackpad button or press the Media Eject key (⏏) or F12 key during startup to eject any disk from your Mac

Recovery Tools: 

Press and hold Command(⌘)-R during startup gives you access to recovery tools, which allow you to format a drive, restore a version of OSX and more.

Start up from the default NetBoot disk image: 

Pressing the N key during startup allows you to access a disk image of OSX stored on a server and install it over a network connection.

Apple Diagnostics or Apple Hardware Test Tools: 

Press the D key during startup will let you access either the Apple Hardware Test or Apple Diagnostics depending on which Mac you're using.

Start up in Target Disk Mode: 

Press and hold the T key during startup allows you to boot into Target mode. Turning your computer into an external drive for another Mac (same version of OS X with similar hardware configurations) and they must be connected via FireWire or Thunderbolt.

Reset parameter RAM (PRAM): 

There may be times when you need to reset your PRAM where a lot of your hardware settings are stored. Press Option-Command(⌘)-P-R during startup. 

Start up in Safe Mode: 

Safe Mode is useful for troubleshooting Macs as it prevents most software from automatically opening and forces a check of the startup volume and only loads essential OSX items. When you hear the startup tone, immediately hold down the Shift key. Release the Shift key when you see the progress spinning gear indicator. 

Show detailed status messages (verbose mode): 

Verbose mode displays text-based information about the startup process and is useful for troubleshooting issues during startup. Press Command(⌘)-V during startup. 

Start up in single user mode: 

Single user mode gives you an interactive text prompt making it another useful troubleshooting mode when you run into corrupted files or a stubborn application during startup. However, this can lead to more problems then its worth if you don’t know what you're doing so use with caution. Press Command(⌘)-S during startup.