Id vs Class

Question

Hello, so they say that in an html code, the "id" is unique -couldn't be used more than once, and the "class" notion isn't, so you can use it how-much-ever you wanted to.
I basically see this explanation as unclear since I've tried using the same "id" in different tags e.g: <p id="test">...</p> , <div id="test">...</div>. This ended up having both tags affected by the modifications of #test in my css file...How Come? Isn't the ID unique?
I'd like a detailed explanation of the diff. between an ID & a class.

Answer 1

The lessons are quite good too.
https://www.sololearn.com/Course/HTML/?ref=app

Answer 2

https://css-tricks.com/the-difference-between-id-and-class/

Answer 3

Thanks, excellent article!

Answer 4

No problem :)

Answer 5

Yes they are, but not as clear.