What are classes and id? are they the same?(IN HTML AND CSS)

Question

I could not figure out about class es and id and there uses

Answer 1

id is used by only one element, and generally used just for js to single out that one element
class can b used by multiple elements and is used for styling like
.bg-blue {
background-color: blue;
color: white;
}
and u could use that class for any element to b styled that way, in css class is defined as
.class
and id...
#id

Answer 2

*id can be applied once per page
*class can be applied multiple times in a page

Answer 3

in css
class elements are specified with (.)
id elements are specified with (#)

Answer 4

thanks