system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: In html, what is the difference between a div id and a class

And how and when would you use one over the other?

I cannot wrap my head around this concept.

Thanks!
gpt
An id is a unique thing - an identifier for a specific object. A class is a name for multiple similar/identical objects. 

Imagine you create a long list of <person id="x" class="y"> tags. Values for id are social security numbers - values for class are "student", "assistant", "professor". Obviously you can have multiple persons that are students, but only one person should have a specific social security number.