It "is the free, online availability of scholarly research outputs. OA deals primarily with access to peer-reviewed scholarly articles, for which authors receive no financial compensation." from http://www.carl-abrc.ca/doc/CARL_OA_flyer_e.pdf
Open access journals use a funding model that does not charge readers or their institutions for access. from DOAJ - Definitions
Self-archiving is "the act of (the author's) depositing a free copy of an electronic document online in order to provide open access to it..... for the purpose of maximizing its accessibility, usage and citation impact," and sharing their research. The document will include usually "peer-reviewed research journal and conference articles, theses and book chapters, deposited in the author's own institutional repository or open archive." from https://en.wikipedia.org/wiki/Self-archiving
Authors who have received a grant from the Tri-Agencies (CIHR, NSERC, or SSHRC) are required to either publish open access or self-archive their work. However, all grant recipients are still encouraged to self-archive their peer-reviewed manuscripts - this must be done within 12 months of publication, and the manuscripts must be self-archived in either an institutional or disciplinary repository. This could include the BCIT Institutional Repository, PubMed Central, arXiv, etc., but does not include commercial academic social networks (Researchgate, Academia.edu) or personal webpages (blogs, portfolios). The requirements state that it must be a peer-reviewed manuscript, which includes post-prints and publisher versions, but excludes preprints.
Before self-archiving, check to see what the requirements and regulations are from the journal you are publishing in. Some journals do not allow you to self-archive at all, some have embargo periods before you can self-archive, some limit where you can self-archive, and some restrict the version of the manuscript that can be self-archived. If you want to self-archive, consider this factor as you are considering which journal to publish in.
Here are some useful sites: