Tomcat is purely a JSP/Servlet container. If that does not mean anything to you read the J2EE specification.
JBoss is a J2EE application server (and can be configured as other things too) which bundles and uses Tomcat as its implemetation of the J2EE JSP/Servlet specification.
If you need the full J2EE fucntionality (EJB's, JAAS, CMP - refer J2EE spec) JBoss will do. If you only want to run JSP/Servlets you can use Tomcat as a standalone and you don't need JBoss, although it will also do the job.