Just that–a simple way, using they keyboard, to bring different windows to the front/make them active.
Within a single application, cmd-tilde (the ~ key next to Z) cycles between open windows. Or use the f10 function in Exposé to show all windows in a single app, then use the arrows keys to move between them and space or return to select. Esc cancels and returns you to your original window.
For all applications, use the f9 function of Exposé (show all open windows in all apps) and use the same keys to navigate/select.