An application server is an engine designed to host 3rd party code (home grown or from vendors). The code is then made available via the network. The server typically provides basic multi-user functionality (security, administration, clustering, session or connection management, transports, etc.)

Often, application servers are platform specific, such as a J2EE application server. In these cases, the servers also have many of the features required by the platform (JMS, JDBC drivers, etc.) Platform specific application servers have been extended to provide SOAP Server capabilities. In this way, the server doubles as a means to offer both open and proprietary interfaces. Typically, the proprietary method is used for homogenous applications and the open method is used for heterogeneous connectivity.