Purpose

A button initiates an action when the user clicks it.

Guidelines

Use title style capitalization for the label.
Buttons are not dynamic: their icon and label should not change depending on the context. Toggle buttons are a special case where a button has a state.
Do not initiate an action on right-click or double-click.
Prefer using icons on buttons only for OK, Apply or Cancel like actions. Passive actions like those in the "System Settings => Application Appearance => Fonts" do not have icons.