The term Application Server typically symbolists a kind of Java program supporting servers. The business layer programming is deployed in the application server and the server processes the requests, access data, business logics and the final response to the user. Application Server facilitates the simultaneous access from various locations to the same program. Individual threads of application server, handles each requests and forwards for process and response to the same requesting user. Application Server handles the Garbage Collection process which is the destroyal of unused objects, Connection pooling which is the reuse of idle database connections, session time out of user in idle timings and many more such functionalities which are required for the stable execution of the hosted java application.
There are plenty of application servers available in the software industry, in which the most used are Jboss and Glassfish which are open source application servers and Weblogic and Websphere which are commercial application servers from BEA (Oracle bought BEA) and IBM. Glassfish open source application server is built by Glassfish community which includes developers from various companies backed by Sun Microsystem and Oracle Corporation.
The main advantages of application servers are data integrity, performance, transaction support, etc. with all these merits enterprises can focus on core business.