Webserver typically is a HTTP server. It stores the static resources such as GIF, JPEG and other image files, stylesheets, and HTML documents. It forwards the request to one or more application servers. It can run on a different box than the application server.
WebContainer is a part of the application server that hosts dynamic components such as
Servlets, JSPs and other
Java classes. It may inturn talk to the
EJB container, or a database or a legacy system etc. It talks back to the client through the Webserver.
It is quite possible that the functionality of a web server and a web container is satisfied by the same software. Many of the app servers available today such as WebSphere from IBM of IPlanet from Sun offers the combined functionality and can be used both as a web server and a web container.
Hope that helps.