Application servers, or "appservers" execute the operations necessary to complete transactions and other interactions between end users an a business's backend databases and applications. Application servers provide functionality such as load balancing, database access classes, transaction processing, and messaging.
For tiered applications, best practice calls for separating out this application processing from the actual dishing-up of web pages, which is done by a webserver operating in front of the appserver. Appservers, therefore, are key publishing engines in the CM space. Some CM products embed their own application server, while others take advantage of 3rd-party appservers