All submission to the repository are made available under the terms of the Creative commons Attribution-Noncommercial-No Derivative Works 3.0 License.
Open Access is the default status of a submission. Nevertheless, one can fill a waiver asking for an embargo period before his submission becomes accessible.
Usually when you publish an article you sign an agreement with the publisher giving him most of your rights.
Some publishers allow submissions of pre-prints and post-prints to Institutional Repositories. Others may approve the submission of the final pdf version.
SHERPA/ROMeO website is a place where you can check the policy each publisher has.
It is worth to mention that one must always negociate his rights with the publisher and stress on getting a permission to publish in an institutional repository whenever possible.