cin (pronounced "see-in")

an istream class object tied to standard input, cin stands for console input.
int in;
cin >> in;

cout (pronounced "see-out")

an ostream class object tied to standard output. cout stands for console output.
int ou=4;
cout << ou;

cerr (pronounced "see-err")

an ostream class object tied to standard error. cerr stands for console error.