An index is a list of things used to guide users.

In connection with search engines, the index is a list of web pages whose keywords are relevant to the search query. There are two types of indexes, the organic index and the paid index.

Organic Index

The organic index is created by the search engine's own algorithm after crawlers analyzed the websites. The higher the ranking of a website is, the higher it will be positioned in the search results.

The paid index is the payable version of the organic index. Webmasters can use it to advertise their website. The results from the paid index are usually displayed above the results from the organic index.

An index can help users to orientate themselves, they will find quicker what they are looking for.

SEO means trying to optimize a website for the organic index and to improve the positioning of the website in the SERPs.