Difference between "while" loop and "do while" loop

What is the difference between while loop and do while loop. I used to think both are completely same.Then I came across following piece of code:

do {
        printf("Word length... ");
        scanf("%d", &wdlen);
    } while(wdlen<2);

This code works perfectly. It prints word length and tascans the input. But when I changed it to

while(wdlen<2){
        printf("Word length... ");
        scanf("%d", &wdlen);
    } 

It gives a blank screen. It do not work. So there is some functional difference between both loops. Can anybody explain it?

Is there any other difference in these two?