In addition to what Alexis said, GlassFish does have the capability, via
the "docroot" directory, to serve up HTML pages, images, and other files
that are not packaged into a web application. This may be what is giving
you the feeling that it seems that it is also a web server.
Where a true web server is different though is that most web servers
also provide capabilities to do many other functions via proprietary
plugin interface such as CGI, server side scripting, load balancing,
reverse proxy, filtering, file-based access control, etc. As a Java
application server, GlassFish provides an environment for running Java
EE applications. The only overlap with a web server is in the ability
to serve up files.
Tom