Here's an example that's visual: You are putting interaction widgets on the screen- they display some text, perhaps some other visual indicator, and have a button or text area or slider or something on them (there are some of each). Your base class has the setup: the window/panel/whatever-your-language-calls it, the place where the text goes, and the place where the variable widget goes. All of these widgets inherit the base class, and only differ in the user response portion. This makes it very easy to change the interaction style.
