Sitecore search is built on top of Lucene, providing the ability to create very fast searching capabilities for your site. Instead of querying against a centralised database (such as Sitecore's SQL DB), it queries Lucene index files which are stored on the physical file system of the web-server. Sitecore provides a layer of abstraction over the Lucene.NET API including a LINQ provider which makes writing Lucene queries a simple & familiar process for .NET developers. Sitecore ships with some standard indexes configured which you can extend or define your own. You can also opt to use SOLR; a centralised & scalable platform built on top of Lucene.