Can I use any other words apart from the #Intro...for instance <div id=".example" >

Answer 1

#Intro selects a single element by its ID called Intro

your second try will fail cause its the wrong syntax. if you wanna select all elements from type div with class example which your second try looked like, use following syntax :
div .example {} and yes you can set an elements ID or class in a way you like e.g. test, try or abcdefg. But the way you set them is a topic of Html and not CSS

Answer 2

by the way.... dont write id=".example" the "." operator is used to identify classes in CSS