Generally speaking, booting up a Mac device is a breeze. It's quick, it's easy, you just press the power button and usually, ten or so seconds later, you have a login screen and you are away.
Every now and then, however, you have to press a button when booting to troubleshoot an issue or boot from a separate device.
Below is a list of mouse/keyboard shortcuts that may be worth keeping hold of in the event you run into trouble and need to use them:
HOLD 'C' KEY DURING STARTUP - this will force your machine to boot from a CD/DVD
HOLD 'X' KEY DURING STARTUP - this will force your device to boot from OSX. It doesn't matter which disk has been specified as the startup disk.
HOLD 'T' KEY DURING STARTUP - this allows your device to use any Mac with a Firewire port as the source for boot.
HOLD 'D' KEY DURING STARTUP - this allows your device to boot from the Apple Hardware Test DVD
HOLD OPTION KEY DURING STARTUP - this brings up the Startup Manager that allows you to select which disk to boot from
HOLD SHIFT KEY DURING STARTUP - this allows you to boot into Safe Mode
HOLD COMMAND + 'S' DURING STARTUP - this boots into a mode called Single User mode. This is for trying to troubleshoot complex hard drive issues.
HOLD COMMAND + 'V' DURING STARTUP - fancy seeing exactly what is happening when your Mac is booting? This boots in verbose mode.
HOLD COMMAND + 'R' DURING STARTUP - this allows your Mac to use the Recovery partition
HOLD DOWN THE MOUSE OR TRACKPAD BUTTON OR HOLD DOWN THE MEDIA EJECT KEY (⏏) OR F12 KEY. - This allows you to eject a DVD or CD
HOLD DOWN OPTION + COMMAND + 'P' + 'R' - this allows you to reset the parameter RAM. This returns to its default settings for video settings, speaker volume, time and date and DVD region settings.