An index is a structure whose function is to order and sort data in order to find them more quickly. By analogy, the computer index has more or less the same purpose as the one found in a library, except that instead of book titles, URL and website contents are listed. 

Indexing is intimately linked to the visibility of a website on the Internet. Ensuring the proper indexing of a site means checking that nothing prevents search engine robots (such as Google) from visiting and ranking the site in their respective indexes. A good indexing also avoids that useless pages are found in the search results.

The indexing of a web page can be not only automatic – for example, by Google, which uses an index for the referencing of Internet sites at a given moment in time and the display of results in its search engine – but also manual.