Newbie asks: "What are the "#" and "."?

Excuse my newbieness but what do the “#” and “.” mean at the beginning of a line in an external css file? What does each of them do and how are they different? I have searched but cannot find the (simple) answer to this.

the # is an ID selector: it selects elements with a certain ID like <DIV id=“test”></DIV> will be found with the selector #test

the . is a Class selector: it selects elements with a certain Class like <DIV class=“test”></DIV> will be found with the selector .test

That’s great Gar onn. That is clear and concise. Thank you for your help.