A SharePoint Search service application is used to create indexes and to service search requests. The application crawls the contents of websites and creates an index, which is used to serve the client requests for search.
A Search service application contains the following components:
Admin
This component stores the configuration data and security descriptors. Each Search service application instance can only have one Admin component.
Crawl (Crawl Server or Indexer)
This component crawls (accesses and catalogs) the associated website contents and creates the index. It propagates the index files to Query Server. After propagating all the index files, they are removed the Crawler.
Query (Query Server)
This component is are responsible for serving search queries. A Query Server is a server that runs one or more Query components. Query Servers store the full or partial search index.