There are many reasons why it is important to cite the resources that you consult when researching a paper. The most important of these are:
There are four major citation styles: American Psychological Association Style (APA), Chicago (or Turabian) Style, Council of Science Editors (CSE) Style, and Modern Language Association Style (MLA). Which citation style you should use depends upon the subject area of the class and the preferences of your professor.
For your GMO assignment, you will be required to cite your sources using the APA style.
A short video from Western Washington University about why we cite.