An index is generally a directory in a specific order which is used for orientation purposes. In terms of search engines, an index is the listing of webpages which is output by the search engine in response to a user search request.
The listing which gets displayed after you enter a specific search request is called SERPs (Search Engine Result Pages). The search result lists form the index interface which is visible to the users. The actual index is stored on numerous servers belonging to the search engine operator. There are millions of websites that are linked to specific keywords and keyword combinations - similar to alphabetical indices or tabs.
The exact algorithms which are used to form such an index are largely unknown or secret, but it is clear that website crawling is the first step to produce the index and that websites get tagged with a lot of information, such as PageRank, before being output in response to a user request. One way for a new website to be included in a search engine index, is to register with the search engine operator.
If you enter a search term such as “search engine optimization” in the Google or Bing search bar, you want to be shown websites relevant to this term. You will get two types of indices, the organic and the paid results. The paid index is differentiated on Google (the search engine with the largest market share) with yellow highlighted results and their placement on the right or top of the screen. Other search engines such as Yahoo, Bing, or Ask.com color-code the paid index as well or designate it with words (for example, ad) so that it can be distinguished from the organic index.
The organic index shows the ranking of a website as calculated by the search engine. It is determined by a proprietary search engine algorithm after the website is crawled by the search engine spider. The higher up the page appears on the search results list, the higher is its ranking. The goal of the SEO industry is to have a website rank as high as possible on the list through various optimization steps, whereby the search engine’s guidelines are important to follow.
In contrast to this is the paid index. Webmasters can have their website appear as a paid ad above the organic search results. The search engines offer a number of specialized payment programs. For Google it is Google AdWords, for Yahoo and Bing they are BingAds in the Yahoo Bing Network. The ranking of results is not done by an automated process, but rather by the payment programs and bidding on keywords in the paid index. The higher the position in a paid index, the higher the bid on appropriate terms or combinations of terms.
If you set up a website newly, you have different options to be added to the index. Google can be requested to crawl a website, for example.
Once Google has crawled a website for the first time, the indexing status can be accessed. You will need a Google account in the Google Webmaster Tools for this purpose and to submit your website to Google. An XML format sitemap will provide Google with more information about the structure of a website and is recommended especially for larger web projects.
A variety of means can be chosen to raise a website on the index. In terms of the paid index, as already mentioned, you have to place bids on genuine keywords or longtail combinations. For the organic index, the procedure is much more complex and a very important part of search engine optimization.
The second index ranking factor is website relevance. The better its content answers a searcher’s request, the higher it is likely to rank in the index. Relevance can therefore be equated to helpful user information. There are various ways to change the relevance of a website, including quality content, clear infographics or helpful videos which provide complete answers to user questions.