cout, cerr and clog are objects of class ostream that represent the standard output, the standard error and standard log streams respectivelly. They correspond to the cstdio streams stdout and stderr.
  By default, standard output provided by cout, cerr and clog is directed to the screen although this may vary depending on the environment.
  As ostream objects they perform output operations as streams of characters, to which we can write formatted information using operator << or unformatted data using member functions like put, and write.
  Please refer to the C++ tutorial section 1.4: "Communication through console" and the ostream reference for more information on cout, cerr and clog.

See also.
  cin
  iostream library