Search engines perform several activities in order to deliver search results.
Crawling - Process of fetching all the web pages linked to a website. This
task is performed by a software called a crawler or a spider (or Googlebot,
in case of Google).
Indexing - Process of creating index for all the fetched web pages and
keeping them into a giant database from where it can later be retrieved.
Essentially, the process of indexing is identifying the words and expressions
that best describe the page and assigning the page to particular keywords.
Processing - When a search request comes, the search engine processes
it, i.e., it compares the search string in the search request with the indexed
pages in the database.
Calculating Relevancy - It is likely that more than one page contains the
search string, so the search engine starts calculating the relevancy of each
of the pages in its index to the search string.
Retrieving Results - The last step in search engine activities is retrieving
the best matched results. Basically, it is nothing more than simply
displaying them in the browser.
Search engines such as Google and Yahoo! often update their relevancy algorithm
dozens of times per month. When you see changes in your rankings, it is due to
an algorithmic shift or something else beyond your control.
Although the basic principle of operation of all search engines is the same, the
minor differences between their relevancy algorithms lead to major changes in
the relevancy of results.