Open Access literature is digital, online, free of charge, and free of most copyright and licensing restrictions. What makes it possible is the internet and the consent of the author or copyright-holder.
In most fields, scholarly journals do not pay authors, who can therefore consent to Open Access without losing revenue. In this respect scholars and scientists are very differently situated from most musicians and movie-makers, and controversies about Open Access to music and movies do not carry over to research literature.
Open Access is entirely compatible with peer review, and all the major OA initiatives for scientific and scholarly literature insist on its importance. Just as authors of journal articles donate their labor, so do most journal editors and referees participating in peer review.
From Peter Suber's Very Brief Introduction to Open Access.