To open a Terminal window on OS X, click on the Terminal icon in the
dock: 
Since OS X operates on a Unix platform, you can use the following basic
Unix commands in the terminal window:
| ls | List - lists the files and directories in the current directory. |
| cd directoryname | Change Directory - changes directory to any directory inside the current directory. Use the ls command to determine which directories are there, and the cd command to get into one of them. |
| cd .. | Change Directory - changes directory to the directory in which the current directory resides-or "moves up" one level. Once you have gotten into a directory, use cd .. to get back out. |
What is the Terminal, anyhow?
The Terminal is simply a window that allows you to type commands directly
to the computer. This is in contrast to using the graphical interface,
which allows you to give commands to the computer by dragging and dropping,
clicking on buttons and menus, etc.