These  are keyboard modifiers that are held down when booting the computer.


Safe Mode

Shift

System will test startup procedures and limit launched processes.

Single-User Mode

Command-S

Starts only core functions and displays command line.

Verbose Mode

Command-V

Shows the startup process.

Target Disk Mode

T

Mounts another computers drive through firewire or Thuderbolt ports.

Boot from CD/DVD

C

Start up from a bootable CD or DVD.

Select Boot Drive

Option

Will display a list of bootable drives.

NetBoot

N

Boot from server.

OS X Recovery

Command-R

Will display OS X Utilities.

OS X Internet Recovery

Command-Option-R

Force startup in Internet Recovery Mode.

Hardware Test

D

Starts from diagnostics DVD or from Recovery partition.

Apple Diagnostics

Option-D

Starts hardware test from Apple servers over internet.