Authors retain copyright of their work, unless they transfer all or specific rights with an agreement, typically with publishers. Author transfer agreements are important opportunities to negotiate with your goals in mind.
Authors who retain copyright can designate acceptable uses with Creative Commons licenses. These licenses clarify limitations and permissions for publication and use of your work.
It is important to know what rights you retain to your work after publication: Are you able to share a copy of your manuscript with friends? Can you legally post a copy on your webpage? The website SHERPA/RoMEO records author rights information for journals and publishers, allowing you to find out what rights you will have before you even submit the manuscript.