I guess this is the day to post intros! Peter Suber, at Open Access News, has published a very concise introduction to the concept of open access. A Very Brief Introduction to Open Access is a shorter version of his Open Acecss Overview. Both are great documents, though the former is certainly easier to digest for those of use with short attention spans.
If you find the subject interesting, Jessamyn also pointed us to some good related resources this week: Removing the Barriers to Research: An Introduction to Open Access for Librarians (again by Peter Suber) and How and Why To Free All Refereed Research From Access- and Impact-Barriers Online, Now (from HEP Libraries Webzine).