When you create a work and record it into a fixed medium, it is immediately and automatically copyrighted. As the creator of the work, you own the copyright and all the rights that go along with it. Many traditional publication agreements, however, ask the author to transfer all rights--including copyright--to the publisher. Depending on the agreement, you may no longer be able to use your work in future publications or teaching, distribute your work to colleagues, or post your work in an online repository. In essence, you have given up your exclusive rights to your own research and must rely on Fair Use.
Are you aware you might be breaking copyright law when you post your published papers on your personal web page? Do you know if your publisher will let you add an addendum to retain your rights?
An author's addendum helps you retain the essential rights to your work, while still giving the publisher the right to put your work in their journal. They are are designed to attach to and supplement an existing transfer of copyright agreement.