Buttons up or down?

For years I have been using, apparently incorrectly, the terms "button-up" and "button-down" interchangeably. (Who cares which way you button your shirt, anyway?) However, as it turns out, a button-up shirt is simply a dress shirt; a button-down shirt, on the other hand, is a shirt with a button-down collar, and is considered "a more sporting style". Who knew?