Many Web sites need to be updated frequently so that the information they present doesn't become stale. But the presence of new information may not be obvious to readers unless you make a systematic effort to inform them. If items that appear on your home page menu are updated, you could place a "new" graphic next to each updated item. If, however, your site is complex, with many levels of information spread over dozens (or hundreds) of pages, you might consider making a "What's New" page designed specifically to inform users of updated information throughout the site.
Every Web page in a corporate or institutional site should carry a revision date that is changed each time the page is updated so that users can be sure they have the latest version. Many readers print pages from the Web. Without a revision date, your readers have no way of telling whether the page they printed is current or outdated.