A toggle button is a button which stays down when clicked once and goes up when clicked a second time.
When to use
Use a toggle button to indicate a state rather than an action. For example a word-processor would use toggle buttons to indicate the state of "Bold", "Italic" or "Underline" formatting.
Do and don't
Do not change the icon or label of a toggle button. The fact that it is pressed down is enough.
Since a toggle button represents a state, in most cases the label should be a noun rather than a verb.