The SharePoint Search Service Application is split into components:
The Search Administration Component is responsible for keeping track of component topology.
The Crawl Component spiders content sources, such as SharePoint Web Applications, User Profiles, file shares and Exchange public folders.
It discovers content and sends that to the Content Processing Component, which parses the incoming stream of documents and properties, and sends them to…
The Index Component, which builds flat file indices on the local filesystem of the server, not in a database.
The Analytics Component is invoked during crawls, and processes links between documents and click events, for analytics reporting.
From the other end, for instance when a user submits a query, it’s sent to the Query Processing Component for parsing.
This sends the query to the Index Component to run the query itself, and look up the results from the index.
The Query Processing Component security trims the results before sending them back to the browser.