In C++, cin is an object of class istream that is used to accept input from the standard input device, which is usually the keyboard. It is defined in the iostream header file and is used in combination with the stream extraction operator (>>) to read input from the console. The cin object can be used to read different types of data, including integers, characters, and strings. When cin is used to read input, the program waits for the user to enter some sequence with the keyboard, and the characters introduced using the keyboard are only transmitted to the program when the ENTER (or RETURN) key is pressed. If the user enters something that cannot be interpreted as the expected data type, the extraction operation fails, and the program continues without setting a value for the variable, producing undetermined results if the value of the variable is used later.