Apache Tomcat is a Servlet container developed at the Apache Software Foundation (ASF). Tomcat implements the Java Servlet and the JavaServer Pages (JSP) specifications from Sun Microsystems, and provides a "pure Java" HTTP web server environment for Java code to run.
In our world of Java, it's more common to refer to J2EE as a more complete
reference of what an application server can and should include, of which
Tomcat only implements a part, while the complete download of JBoss
(including Tomcat) conforms to the full J2EE specification, and hence is a
more "complete" application server.