Terminal is the command prompt for Mac OS X. Its very useful for making changes to your OS which mac only allows through terminal. Of course learning the all the commands doesn't really make much sense and then of course, you can just copy paste it from here. Here are some cool ones -
1. 2D Dock -
defaults write com.apple.dock no-glass -boolean YES
Press Enter.
killall Dock
2. Play Star Wars Episode IV -
telnet towel.blinkenlights.nl