"In short, Open Access is the free and unrestricted online access to outputs of scholarly research." Open Access and Digital Scholarship Blog, Imperial College London, http://wwwf.imperial.ac.uk/blog/openaccess/about-the-open-access-blog/
Although Open Access can also apply to books, music, images, and more, open-access journals are particularly prevalent in academia.
OA journals are available online for everyone to read for free, instead of being accessible only to paid subscribers.
Many OA journals employ scholarly peer-review and rigorous editorial processes, just as their traditional commercial counterparts do. Therefore the quality of the research product is not diminished, but access to that product is more open to students and researchers around the world.
Explanation of publishing models: In general,
Usage of these terms may vary, so always read a journal's policies carefully, or consult the Directory of Open Access Journals for clear information about a journal's OA publishing and author fees.
"Removing price and access barriers to articles allows the broadest possible audience and reach for scholarly works, which could increase the visibility [of] the works and their impact."
Introduction to Open Access, Georgetown University, http://www.library.georgetown.edu/scholarly-communication/introduction-to-open-access
"Accelerated discovery. With open access, researchers can read and build on the findings of others without restriction.
"Public enrichment. Much scientific and medical research is paid for with public funds. Open Access allows taxpayers to see the results of their investment.
"Improved education. Open Access means that teachers and their students have access to the latest research findings throughout the world."
Quoted from https://www.plos.org/open-access/ - "PLOS (Public Library of Science) is a nonprofit publisher and advocacy organization founded to accelerate progress in science and medicine by leading a transformation in research communication."
The following infographic from bepress highlights some of the impacts of open access for different stakeholder groups.
(click on the image to view it full-size in a new window):
Your institution and/or the agency funding your research may have mandates regarding open access (especially if you are funded by a government agency). Read the terms of your grant funding and plan in advance if your publications must be openly available.
The tools below can help you to identify mandates from your institution and/or funding agency.