Terminal
OS X is based on Unix. If you need proof — or if you actually want to operate your Mac as the Unix machine that it is — Terminal is the place to start.
Because Unix is a command-line-based operating system, you use Terminal to type your commands. You can issue commands that show a directory listing, copy and move files, search for filenames or contents, or establish or change passwords. In short, if you know what you’re doing, you can do everything on the command line that you can do in OS X. For most folks, that’s not a desirable alternative to the windows-and-icons of the Finder window. But take my word for it; true geeks who are also Mac lovers get all misty-eyed about the combination of a command line and a graphical user interface.
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