Classes and ID's

Question

When should I use a "class" and when should I use"id"?

Answer 1

they can be used in place of others but id are meant to be given to uniquely identify elements be it pointing them in scripts like javascript or using links or applying specific styles .

on the other hand, classes are meant to be given to many elements which require same style for example, you can place several buttons the ok button cancel button next etc. there we generally use id different and unique but will give same class to style button.