Indexes (alphabetical lists of content) have a similar level of importance online as offline although many work behind the scenes online instead of being manually searched by users. A website might index keywords and content in articles to allow faster and more accurate searching of articles on the site.
More sophisticated search engines (like Google) will build huge web indexes as well as cached content to enable quick searching and the technology has moved on considerably from users scrolling through lists of letters at the back of books. The principles are the same, however and some larger or academic sites will still provide their users with a more old fashioned index.