Complex sentence: Macs have a key for compatibility with programs that expect a more traditional keyboard layout, and it is especially useful when using a terminal, the X11 (a Unix environment included with OS X as an install option) or MS Windows.
Split into simpler sentences: Macs have a key for compatibility with programs that expect a more traditional keyboard layout. It is especially useful when using a terminal, the X11 (a Unix environment included with OS X as an install option) or MS Windows.