What's the difference between an id and a class?

Answer 1

Any element can have a class or an id.

A class is used to reference a certain type of display, for example you may have a css class for a div that represents the answer to this question. As there will be many answers, multiple divs would need the same styling and you would use a class.

An id refers to only a single element, for example the related section at the right may have styling specific to it not reused elsewhere, it would use an id.

Technically you can use classes for all of it, or split them up logically. You can not, however, reuse id's for multiple elements.