Peter Suber has written a helpful short Open Access overwiew for those who are new to the concept:

http://www.earlham.edu/~peters/fos/overview.htm

A few comments:

Suber gives his text the title: "Open access to peer-reviewed research articles and their preprints"

My opinion is: Open Access is a wider concept.

"Open-access literature is digital, online, free of charge, and free of most copyright and licensing restrictions."

Right.

"The campaign for OA focuses on literature that authors give to the world without expectation of payment."

Right.

"OA focuses on royalty-free literature because it is the low-hanging fruit, but OA needn't be limited to royality-free literature."

Right.

Open Access is a general concept including the creation of a rich public domain (for scholarly and other purposes) by e.g.
- making accessible scholarly and other works (e.g. historical sources) which are in the public domain or should be.

One can e.g. mention XIXth. century scholarly articles/monographs or source editions.

The Berlin declaration is demanding Open Access also for heritage collections ("Kulturgut"). This is not quite popular in the OA community (focussed on journal literature) but it is IMHO very important for the future of the humanities and historical research. I do not appreciate that Suber has omitted this fact. Berlin says: "We define open access as a comprehensive source of human knowledge and cultural heritage that has been approved by the scientific community." (my emphasis).

Postscript
Suber has recently added the following:
"Nor need OA even be limited to literature. It can apply to any digital content, from raw data to music, images, and software. It can apply to works that are born digital or to older works, like public-domain literature and cultural-heritage objects, digitized later in life."