What is CORBA 2? CORBA 3? What does the version number mean, anyhow?

Formally, CORBA 2 and CORBA 3 refer to complete releases of the entire CORBA specification. However, because OMG increments the major release number only when they make a significant addition to the architecture, these phrases become a sort of shorthand for just the significant addition. So, "CORBA 2" sometimes refers to CORBA interoperability and the IIOP protocol, and "CORBA 3" sometimes refers to the CORBA Component Model (even though the term CORBA 3 really refers to a suite of ten specifications!). It's easy to tell, from the context, which meaning an author intends.