The Internet Archive’s Wayback Machine has been archiving the static content of the world-wide web since 1996. With it, it’s possible to see what a web page contained in the past, or view a page that no longer exists.
Website operators can exclude their site from archival or request removal of already-archived pages, so this doesn’t always work (particularly when the author of the page took it down because, for any reason, they no longer wanted people to see it). Also, there’s a delay of “a few months”, so you can’t view a web page from last week.
Even so, the Wayback Machine is an invaluable resource for research and resurrection. You can see it demonstrated in a previous Essentials post that links to something originally published in late 2001.