What is the difference between id and class in CSS, and when should I use them? [duplicate]

Answer 1

As pretty much everyone else has said use ID for one-off elements and class for multiple use elements.

Here is a quick, over simplified example, take HTML and HEAD tags as read

    

        You will only have one left column
    
    

        Only one main content container, but.....
        
You might want more than one pretty paragraph
        
This one is boring and unstyled
        
See I told you, you might need another!
    
    

        Not very HTML5 but you'll likely only have one of these too.
    

Also, as mentioned in other answers ID are well exposed to javascript, classes less so. However modern javascript frameworks like jQuery leverage class for javascript too