All directories on the system are subdirectories of the root directory. The root directory is physically located on the hard drive of the machine onto which you are logged in. Every directory can contain files and/or subdirectories. Any path which begins at the root starts with a / (or slash). A path can lead to directories physically located on other computers. The network system makes this transparent to the user. Any path which begins at your home directory begins with ~/ (or a tilde followed by a slash). Any path which begins with ~ username (or tilde username) starts in that particular username's home directory.