User input: To enable the user to to input a value,use cin in combination with the extraction operator (>>). The variable containing the extracted data follows the operator. The following example shows how to accept user input and store it in the num variables.
For example:

int num;  
cin>>num;  

Note:-As with cout,extractions on cin can be chained to request more than one input in a statement:

int a,b;  
cin>>a>>b;  

Accepting user input: The following program prompts the user to input a number and stores it in the variable a:

For example:

#include<iostream>
using namespace std;  
int main()  
{
     int a;
     cout<<"Enter a number\n";
     cin>>a;
     return 0;
}

When the program runs, it displays the massage "Enter a number", and then wait for the user to enter the number and press Enter, or Return.
The entered number is stored in the variable a.