Crawling, Indexing and Serving is the key processes in delivering search result.
Crawling
Crawling is the process by which Googlebot discovers new and updated pages to be added to the Google index.
Each websites visit bu googlebot it detect link on each page, then adds them to its list pages to crwal. New sites, change to existing sites, and dead links are noted and used to update the Google index.
Indexing
A process of each of the page it crawls by googlebot, will be compile a massive index of all the words it sees and
their location of each page.
Serving
User enter a query, then google machines search the index for matching pages and return the results we believe are the most relevant to the user.
Creadit to : Webmaster Tools Help, 7/22/2011