There should be only one of these given a choice of buttons, it should be the affermative action i.e. takes the user to the next step
These are ancillary choices to the primary action, or persistent functions
These are for functions that are localised to a portion of the screen. They can also be used to save space.
These buttons are deisnged to fit in with form fields.
Like tabs or a group of radio buttons
Top button group
A group of buttons that control parameters for the content below it.
Bottom button group
A group of buttons are normaly aligned to the right to signify the next step in an interaction. The primary button is always on the far right