Primary buttons consist of actions that are essential to whatever you are building. They are set apart by their fill and use of elevation.
Secondary buttons are generally used for less-pronounced actions, along with those located in cards.
Text buttons are generally used for less-pronounced actions, along with those located in cards.
Secondary buttons consist of actions with context. For example dangerous action for deleting.
The button toggle is only styled in Angular Material right now. A non-Angular version is on the road map. View the Angular Material Playground for examples of the styled button toggle.
Button toggles allow multiple buttons to be stacked together on a single line. Button groups are used to group buttons together with similar actions.