a web server was traditionally used to serve up static web resources such as images, sounds, html and so on. then to cope with dynamic content demands began to support cgi, php, servlets and so on.

however, app servers, house "bigger" applications and provide more sophisiticated services that these applications require (security, transactions, database connection pooling)

web servers are best for serving static content and app servers for dynamic, application specific content. that is why is not uncommon to see clusters of appservers fronted by web servers that forward HTTP requests to the appservers for more complex app-specific processing.

on a final note, most app servers come with an httpd server built-in, but i tend to use web servers for serving static content because they have a history of doing it well.