A commonly used definition of Open Access is the free, immediate, online availability of research articles, coupled with the rights to use these articles fully in the digital environment. (From SPARC)
In a traditional publishing model, publishers own the rights to the articles in their journals. Users have to pay to access them, either through institutional subscriptions commonly managed by libraries, or through personal subscriptions or one-time use fees.
More information on Copyright:
An author can publish an open access article in a few different ways: