Here’s a great resource if you want to come up to speed on ontologies and their importance today.

Professor Barry Smith of the University at Buffalo held a two-day course, An Introduction to Ontology: From Aristotle to the Universal Core, in 2009, to introduce ontologies and their applications to both philosophers and computer scientists. It consisted of of eight lectures for which slides and downloadable videos are available. Paul Alexander has also made the videos available in streaming form here if you want to view them without downloading.

The lectures are all either 60 or 90 minutes. Here are links to the streaming videos, thanks to Paul Alexander: