New to Java Servlet
You don't need to know Java Servlet to use ZK, since ZK encapsulates the HTTP requests to higher-level concepts, such as ZUL pages and executions. However, it is helpful if you have some Java Servlet concepts.
Java Servlet Tutorials
A Web container is a server to serve HTTP requests. A software developer who wants to add dynamical content usually implements a so-called Java Servlet.
A Java Servlet is a Java class responsible for processing HTTP requests and then generating the corresponding HTTP response (usually in the format of HTML), based on the requirement of your applications.
A Web container is also known as a Servlet container. Tomcat is one of the most famous Web containers.
Rich Java offers a good tutorial about Java Servlet.