Class vs ID

The difference between ID and class is that an ID selector can be called only once in a document, while a class selector can be called multiple times in a document. The second difference is that ID can be called by Javascript's getElementByID function. 

There is no hard rule on when to use ID and when to use Class. My suggestion is to use class as much as possible for maximum flexibility, with the only exception being when you want to use Javascript's getElementByID function, in which case you need use ID. 

Class and ID names are both case sensitive. For example, .classone and .ClassOne are two different classes.