cin is an object of class istream that represents the standard input. It corresponds to the cstdio stream stdin.
  By default, standard input is read from the keyboard, thus cin is generally used to get information directly from the user.
  As an istream object interprets input as a stream of characters, from which we can retrieve formatted data using operator >> or unformatted data using member functions like get, getline and read.
  Please refer to the C++ tutorial section 1.4: "Communication through console" and the istream reference for more information on cin.

See also.
  cout, cerr and clog
  iostream library