Buttons can do an action, display a menu, or both:
Buttons have showLabel=false so text is not displayed. Should have title attribute displayed on mouse over
The button CSS as well as the icon CSS can change on toggle
Short button, tall buttons, big buttons, small buttons... These buttons size to their content (just like <button>).
Dojo users can customize styles. Here's an example:
clicking the buttons below will change the buttons above