Search engines perform several activities so as to deliver search results
Crawling is the process performed by search engine crawler, when searching for relevant websites on the index.
Process of fetching all the online pages linked to an internet site. This task is performed by a software, called a crawler or a spider (or Googlebot, just in case of Google).
Process of making an index for all the fetched sites and keeping them into an enormous database from where it can later be retrieved. Essentially, the method of indexing is identifying the words and expressions that best describe the page and assigning the page to particular keywords.
Indexing generally refers to getting your web pages (or other various forms of content from PDFs to videos) indexed in Google & other search engines. A search engine will crawl your website in search of content to add to its index.
You can think of an index as a database of content deemed worthy enough to display as answers for search queries. Technical issues with your site can cause pages not to be indexed, as well as other low-quality indicators including duplicative content, stolen content, shallow content, bad site signals, bad user experiences, malware, etc. Google also maintains a supplemental index, which is a secondary database containing pages of lesser importance (and unlikely to show on searches unless you force Google to show them).
When an inquiry request comes, the program processes it, i.e. it compares the search string within the search request with the indexed pages within the database.
4) Calculating Relevancy
It’s likely that quite one page contains the search string, therefore the program starts calculating the relevancy of each of the pages in its index to the search string.
5) Retrieving Results
The last step in program activities is retrieving the simplest matched results. Basically, it’s nothing quite simply displaying them within the browser.
Search engines like Google and Yahoo! often update their relevancy algorithm dozens of times per month. Once you see changes in your rankings it’s thanks to an algorithmic shift or something else outside of your control.
Although the essential principle of operation of all search engines is that the same, the minor differences between their relevancy algorithms cause major changes in results relevancy.