ID vs CLASS in CSS. Please explain in detail with example

Question

I have heard that ID is unique and can only be used once in a page, but its working fine when used over multiple times on a page. Please let me know the purpose of ID and hows i

Answer 1

An ID is used to reference certain elements when using JQuery / Javascript etc, so technically if you wish to utilise these features then IDs on your page should be unique.

If you are only bothered about styling then it really isn't a huge issue, although it isn't valid practice.