Web Servers (Web Servers are dedicated to serving web pages. Their functionality is extended by integrating CGI, scripting engines, and Servlet containers, to provide processing power and access databases, mail servers, and directory systems.)
Servlet Containers (These servers provide
Java Servlet support as well as being web servers themselves. They are frequently integrated with a standard web server.)
Application Servers (Application servers provide specialized functions such as EJB, messaging, and transaction monitoring. Application servers operate in the middle tier supplying the business logic and heavy duty processing for a larger application.)
taken from
http://www.dynamic-apps.com/servers.jsp