SharePoint 2010 Server Roles
Depending on the size of your SharePoint 2100 deployment, you may have one or more SharePoint servers assigned to serve specific purposes, or roles, including these:
Web server: This server (also known as a front-end Web server) hosts all Web pages, Web Parts, and Web services used when your server farm receives a request for processing.
Application server: This server hosts the service applications running in the farm, such as Visio Services.
Database sever: This server stores most of the data associated with a SharePoint 2010 implementation — including configuration settings, administration information, data associated with the service applications, and user content.
Query server: This server is responsible for querying the index, finding the matching content, and then sending the content back to the Web servers for presentation to users.
Crawl server: This server crawls (accesses and catalogs) content sources and then propagates the results to the query servers. The crawl server uses a crawl database to store the URLs of all sources crawled.
Roles aren't assigned directly to a server; instead, a server assumes a specific role in your configuration depending on what components it has installed, the services it runs, and its location within your server farm.