Indexing is the implementation of multiple methods to index a website or the entirety of the Internet. Search engines use keywords and metadata while individual websites are more likely to use a standard index style found similarly in the back of a textbook. 
A-Z Index Edit
An A-Z index is mostly implemented by an individual website. This style of index outlines a websites active pages allowing users to find a specific page in the site potentially much faster than normal browsing would allow. It also allows users to see all of the pages on a site at once which may help a user find new content they were not aware of.
Search Indexes Edit
Metadata is one of the most important parts of using search algorithms to build search indexes. Crawlers use a sites metadata and keywords to help index websites into search results. Metadata is becoming more important in website design as it increases the probability of a site being indexed properly. Search engines need proper indexes for fast and accurate search retrievals. Indexing allows for proper mapping of the internet as a whole and helps connect websites to potential users.