David Hogue (Macromedia):

"For most tasks and steps, there is more to do and show than you can place on the screen at one time, yet all of the information may be necessary at some point. There are several ways to make information visible or available on an as-needed basis and to move among steps—you can employ specific user interface components, which correlate to various information display methods. There are four categories of display methods and interface components: modal/non-modal, interactive/static, conceal/reveal on demand, and progressive disclosure."