Tomcat is a web server and servlet container. WebLogic is a JEE server (which supports a lot more APIs than Tomcat does, like EJB).
With the help OpenEJB -explained in the article Hebert linked to- Tomcat can be made to support EJBs, though.
Good news is the OpenEJB/Tomcat stack mentioned has now been officially Java EE 6 certified as Apache TomEE. So now you can get Tomcat with compliant EJB features.