The Internet Archive Wayback Machine is a service that allows people to visit archived versions of Web sites. The original idea for the Internet Archive Wayback Machine began in 1996, when the Internet Archive first began archiving the web. Alexa Internet, in cooperation with the Internet Archive, has designed a three dimensional index that allows browsing of web documents over multiple time periods, and turned this unique feature into the Wayback Machine.
With Wayback Machine, you can browse through 40 billion web pages archived from 1996 to a few months ago. To start surfing the Wayback, type in the web address of a site or page where you would like to start, and press enter. Then select from the archived dates available. The resulting pages point to other archived pages at as close a date as possible.
You can also put the Wayback Machine right in your browser with a Bookmarklet. When you visit a page that you want to find an old version of, just click the toolbar link and you will be transported to any historic versions at the Wayback Machine.
It is an excellent way to see how different websites looked on different dates in the past. It is also a good way to recover the source code of your site incase of a server crash.
The Internet Archive is not interested in preserving or offering access to Web sites or other Internet documents of persons who do not want their materials in the collection. By placing a simple robots.txt file on your Web server, you can exclude your site from being crawled as well as exclude any historical pages from the Wayback Machine. You can also read more details about indexing in the archive and see how to get your site indexed there.