A Servlet Container is any application that supports the Java Servlet specification. Essentially it is software that accepts and processes network requests and forwards those to a Java Servlet, as well as returning the servlet's response to the original requestor. The most common servlet container is a web server which forwards HTTP requests to servlet containers and sends the response back to the client, usually a web browser.

Some notable servlet containers available include Apache/Tomcat, BEA Weblogic, IBM Websphere, JBoss, Caucho's Resin, Jetty, and Orion.

You can view the latest specification for Java Servlet Container (2.4) here