Classes and IDs are how we create different styles for different elements on the page. IDs are unique identifiers and can only be used once on any one page. Classes can be reused and applied to different elements.
So, for example, this is wrong:
But this would be right:
You can also assign classes to different elements:
(How useful this is would depend on the styles attached to the classes.)
IDs are identified in CSS by
# in front of the name, while classes have
. in front of them, e.g.: