There is a very good reference over at w3schools.com that lists all of the classes in Bootstrap, in an article called Bootstrap Classes Reference.

Some of the more common classes you may see are: btn, btn-primary, bg-primary, m-1, p-1, text-white, bg-success, table, table-bordered, table-condensed, table-striped, table-responsive, text-center, jumbotron, hidden, active, alert, and so on.

m-1 sets margin to 1. p-1 sets the padding to 1. For more information you can go to the Bootstrap website and read the article on Spacing.