In a non-Web environment, an application server performs the business logic (the data processing), although some business logic may be handled by the user's machine. See file server
Three-Tier Non-Web Environment
An application server in a three-tier client/server environment provides middle tier processing between the user's machine and the database management system (DBMS).
In a private or public Web environment (intranet or Internet), application servers perform myriad database and data processing operations. There is no limit to the back-end functions an application server can be programmed to do.
Application servers and Web servers may reside in the same computer or be in separate computers. In large sites, multiple application servers and Web servers (HTTP servers) are used. Examples of Web application servers include BEA Weblogic Server and IBM's WebSphere Application Server. See Web server
Application Servers & Web Servers
Both types of servers can perform similar tasks. The Web server (HTTP server) can invoke a variety of scripts and services to query databases and perform business processing, while application servers often come with their own HTTP server which delivers Web pages to the browser.
Java EE Application Server
Application servers have become the middleware for the enterprise as they provide more hooks into many legacy applications. This is a Java EE-compliant application server running only Java and using Enterprise JavaBeans (EJBs) for the business logic. See EJB