Target Disk Mode is a simple and efficient way to move files from one Mac to another. This can be used if your Mac won't boot and you need to get files off of it. Or you could transfer files onto a Mac this way, just as you would with an external hard drive. Here's how to take advantage of this convenient feature built into OS X:
First, power down the computer you'd like to access, and connect it to another computer with a FireWire or Thunderbolt cable. If you're connecting an older Mac to a newer one, you may need this $30 adapter sold by Apple.
Next, turn on the computer and hold down the T key as it boots. The Mac should mount as an external disk on the desktop of the other computer. Then you can transfer files just by dragging them to and from the disk.
When you're done, eject the drive by dragging it to the trash on the other Mac. Then press the power button on the Mac that was in Target Disk Mode, and start it back up as you would normally.
Step by step instructions for this process can be found on this Apple Support Document.