Since Mac OS X is now a Unix-like operating system with a nice fancy "Macintosh" windowing system GUI interface, you can type commands from the keyboard just as with other Unix and Linux systems. To do so, you must access a "Terminal" window which gives you command-line access to a shell. You can do stuff like copy or delete files, create directories, start and stop services (e.g. the web server) all "by hand". This can some times be more efficient or quicker than using a GUI tool, if you are experienced with using a command line and you know the OS X unix commands. (Call me "old fashioned", but heh, I'm sorry, I just don't trust GUI tools because I can't see exactly what they've done.)
To access a "TERMINAL" window under OS X:
You may have a shortcut in the task bar (at bottom of screen) for Applications, or you may not.
If you do, then simply click on the Applications icon in the task bar. ==>Applications ==>Utilities ==>Terminal
If you do NOT have Applications icon in the task bar:
Macintosh HD ==>Applications ==>Utilities ==>Terminal
Note: if you don't have a terminal window icon on your desktop or in the task bar, then make one: click once on Terminal in Utilities folder, then File==>Make alias. Then drag the alias icon to either the desktop or to the task bar.
A terminal ("shell" or "command") window will open up. Click on the window to focus your keyboard on it. Commands are typed in response to the "command prompt", which by default is: "%" or "#" (depending on whether you are logged in as "root" or "admin" user or a regular non-priviliged user). Hitting "ENTER" or "RETURN" on your keyboard submits the command to the shell command interpreter which will execute it. Until you hit ENTER, you can edit the command by Backspace rubout and retype.
Usually, one command per line is used, but multiple commands can be strung together on one line by using a ";" to separate the commands.
Unix I/O redirection applies. Thus, you can "pipe" commands (output of one command is passed on as the input of the next command) and you can direct output to files with ">" and ">>"
You can have more than one shell terminal open at a time, so you can be running several tasks at once. (OS X is a "multi-user/multi-tasking" operating system).
You can exit the terminal window by typing the command: "^d" (that is CTRL-D, hold down CTRL (Control) key and kit "d" key). This is "end-of-file" in Unix. It will close that terminal window. Alternatively, you can kill the terminal window with your mouse.
To learn about the Unix shell commands, ...