Enterprise JavaBeans

EJB, the widely-adopted server-side component architecture for JavaTM 2 Platform, Enterprise Edition (J2EE), enables rapid development of mission-critical application that are versatile, reusable and portable across middleware while protecting IT investment and preventing vendor lock-in.

Well, at least that is what the FAQ on Sun's site says. It is similar to CORBA in ways, but you can only use Java with it (CORBA tries to be language independent). EJB does have the advantage that an EJB container is easier to setup and use then a CORBA orb, but you lose some of the flexability.