Mathematica usually works by taking whatever input you give, and then processing it. Sometimes, however, you may want to have a program you write explicitly request more input. You can do this using
Input and
InputString.
Exactly how
Input and
InputString work depends on the computer system and
Mathematica interface you are using. With a text-based interface, they typically just wait for standard input, terminated with a newline. With a notebook interface, however, they typically get the front end to put up a "dialog box", in which the user can enter input.
In general,
Input is intended for reading complete
Mathematica expressions.
InputString, on the other hand, is for reading arbitrary strings.