Open Access is a publishing and distribution model that provides free, unrestricted access to research outputs such as journal articles and books. Open Access ensures that anyone can access and use these results content is open to all with no access fees.
Check out this introduction to Open Access from PHD Comics.
There are two models of Open Access publishing: Gold Open Access and Green Open Access.
Gold Open Access
Work is published in an Open Access journal that is freely available to all readers. Open journals are often funded through a subsidy from a hosting institution or professional body, or they charge an article processing fee that is paid by the author, academic institution or research sponsor.
Green Open Access
Author chooses to deposit article in an online digital repository allowing readers to freely access and reuse the work. Authors may deposit preprints of their work without obtaining permission from a journal publisher and many journals also allow authors to archive their postprints. It is made available under Open Access regardless of the journal that it was published in.