Open Access is the free, immediate, online availability of research articles coupled with the rights to use these articles fully in the digital environment. Open Access ensures that anyone can access and use these results—to turn ideas into industries and breakthroughs into better lives.
According to UNESCO, a publication is considered fully Open Access if:
Most scholarly open access journals follow a rigorous peer review process identical to those used by traditional subscription journals. One of the basic requirements for journals to be indexed in the Directory of Open Access Journal (DOAJ) is that all articles must go through a quality control system before publication, so checking for inclusion in DOAJ is one quick way to know if a journal is peer reviewed. Most open access journals also have their peer review process outlined on their website.
Open access is an indicator of the type of access, not the level of quality. Many open access journals are highly ranked in their fields, particularly in the sciences. For example, PLoS Biology's impact factor of 12.5 ranks it as #1 in 86 in the Journal Citation Report's biology category. Generally, you can use the same factors to evaluate an open access journal as you would a traditional journal (tip: try out the Journal Evaluation Rubric created by Loyola Marymount University). In addition, for open access journals, you can check for inclusion in the DOAJ and for how well use the journal meets the Committee on Publication Ethics (COPE) Principles of Transparency.
All works are automatically under copyright protection the moment they are created and fixed in a tangible form. When you enter into a publication agreement with a publisher, the publisher usually requires you to sign over all of your copyright (in which case, you can't even use your published work in your own class!). Authors actually retain more or all of their rights when they publish in open access. Open access journals use Creative Commons licenses, which allow for specific reuses of the work with credit given to the original author.
For more common misconceptions about Open Access, check out "Open access: six myths to put to rest" by Peter Suber, published in The Guardian in 2013.