Open Access (OA) is a set of principles and practices through which research outputs are distributed online, free of access charges. Under some models of open access publishing, barriers to copying or reuse are also reduced or removed by applying an open license for copyright. It's aim is to remove the price barrier to academic literature, which is otherwise hidden behind a paywall of academic journals; but it does not always allow those accessing the work the rights to share and adapt the work – this still needs to be done through an open licence.
Dasaptaerwin, CC0, via Wikimedia Commons
This video provides a great overview of traditional academic publishing models and how OA can offer new solutions to access.
Jorge Cham, based on interviews with Jonathan Eisen and Nick Shockey, CC BY 3.0
Gold OA is a publishing pathway that allows the final version of an article to be freely and permanently available online for everyone to read. With gold OA, authors retain the copyright to the work.
Green OA is known as self-archiving, where the author uploads an earlier version of their article in repositories or on their site/social media. Often times the author's copyright is given up with this pathway.