Application server -- an application which processes specific type
of requests and runs perpetually as a daemon/service. It accepts requests
from Web server via some communication protocol, and returns results back
to the Web server. It is always up, and does not need to be started for
each request. It can run on another computer, and finally, there maybe
many application servers running in parallel, and the load can be
spread between them evenly (how do you think Search Engines run?).