Traditional author/publisher agreements are set up in such a way that the author gives away most copyright of articles and books to the publisher, including the right to redistribute, post or re-use the work in any meaningful way. Recent developments (such as the Open Access movement) has lead to significant changes, but the onerous still falls on the author to read and understand the copyright/publishing agreement before accepting it.
These are resources that might make it easier.
Please remember that only a lawyer can provide legal counsel on contracts and all aspects of the law, including copyright. The sources on this page are chosen to provide you with guidance in making your own decisions.