What's the difference between the headline, the title, or the caption and what about the description?
The problem is that the difference depends on which standard is used. You would think that the IPTC would be the standard. IPTC or Adobe? Let's face it, Adobe is bigger and more important. IPTC is used by a few hundred organizations, Adobe (Lightroom and Photoshop CSx) is used by millions of people and I would not be surprised if millions of organizations would be using Adobe.
IPTC stands for the “International Press Telecommunications Council.” It's an organization based in London, UK. It's a study group of news agencies and various news organizations so they use some common standards to exchange data among them. There are problems with the IPTC standards. There are a few versions and they conflict with each other because they renamed some fields and made some other data obsolete. Then the next problem is that they have the “core” and the extensions.