As defined in Wikipedia, an application server is a software engine that delivers applications to client computers or devices. The application server runs your server code. Some well known application servers are IIS (Microsoft), WebLogic Server (BEA), JBoss (Red Hat), WebSphere (IBM).
An application server is a software framework that provides an environment in which applications can run, no matter what the applications are or what they do. .NET Framework along with infrastructure support from window server (e.g. IIS/Web Server) will be an application server.