Difference between getElementById and getElementsByClassName [duplicate]

This question already has an answer here:

• Difference between document.getElementById and document.getElementsByClassName 4 answers

I have written a javascript statement:

var n=document.getElementById("image");

this code doesn't work. But when I change id to class and the statement:

var n=document.getElementsByClassName("image");

it works perfectly fine. Can anyone explain why can't I do the same with id..

Thanks in Advance

NOTE: When I say work I mean variable 'n' has some value stored.

PS: To all those guys who wish to thumbs down this question..I request you to explain the reason before doing so...chill

via Buetman