su is a utility to run a shell or other command as another user (root by default).
su is a utility that runs a shell or a specified command as another user. By default, the other user is root, and su, with sudo, is one of the two common ways of obtaining administrator privileges. Calling su usually requires typing the target user's password.
su originally stood for “super user” (it was only for running a root shell), and became later known as “substitute user” (or “switch user” or a number of similar backronyms) when it started accepting the target user name as an argument.
