There is no one set type of open access. Open Access includes a number of different permissions.
The first distinction is gratis versus libre open access.
The Creative Commons is an excellent resouce for more information on the permissions and types of licenses.
There are serveral other movements promoting the free spread of information.
Open Science and Open Notebook Science - Closely related to open access, however, beyond releasing the results freely available: more from the experiment, paticularly the methodologies, are available as well. Open notebook science can include much more, from the full data sets to the complete lab notebooks for the experiment.
For more information on Open Science, here is an explanation from
Open Source - Open source software is available to use and/or modify by anyone wishing to use or adapt the software.
For more information on OpenSource, here is The Open Source Definition from the Open Source Initiative.
Open Content - For the most part, this is the umbrella of the Open movement. Open content is any creative work/intellectual property released giving the public permission to "Reuse", "Revise", "Remix", or "Redistribute".
For more information on Open Content, here is the definition from OpenContent.org.