Flat buttons are text-only buttons. They may be used in dialogs, toolbars, or inline. They do not lift, but fill with color on press.
Use .btn-outline-* class with any colored button to add button flat.

Raised buttons are rectangular-shaped buttons. They may be used inline. They lift and display ink reactions on press.
Use .btn class with any colored button to add button raised.

A floating action button represents the primary action in an application, it is used for a promoted action.
Use .btn-float class with any colored button to add floating action button.

Use .btn-float-dropdown dropdown to add dropdown action button.

Or .btn-float-dropdown dropup class to flinging them upwards:

A smaller sized, i.e. mini floating action button, is also available. Use .btn-sm class to maintain size of button.