Academic publishers have policies on copyright and self-archiving in place. These policies determine what an author is or isn't permitted to do with her/his publication. Each publisher has set its own policy, so it is worth checking before submitting.
SHERPA RoMEO is a searchable database of journal and publisher policies on self-archiving. It includes which versions of the articles can be self-archived and what (if any) embargo periods are required, with links back to the publisher websites for more details. Use this site to find a journal’s or publisher’s policy for posting author’s manuscripts in institutional repositories such as eScholarship, complying with the NIH Public Access Policy, etc.