In web development terms, "index" usually refers to the homepage of a web site. The index page (index.html, index.php etc) is the page served if no page is specified. On this web site, for example, http://www.webdesigndictionary.com and http://www.webdesigndictionary.com/index.php both load the index page "index.php".
Originally designed by the US Military and called ARPANet, the Internet has grown to a global network of interconnected, smaller networks. The Internet facilitates data transfer and digital communication through, for example, email and the WWW.