Like freedom of speech, academic honesty holds a special place in a community devoted to the creation, preservation, and dissemination of knowledge. For this reason, it is important for you to learn how to acknowledge the contributions of others in your own work and to document properly your reliance on their thinking.
Discovering how to use others' work to advance your own is a key part of learning. Very few of us ever have completely original ideas, and even the greatest scholars build on their predecessors' achievements. Understanding how to incorporate others' points into your own arguments, and how to acknowledge them properly, is one sign of maturing scholarship.
It is also important to understand that failure to know or follow the conventions of documentation and citation—even when inadvertent—is considered a grave breach of academic integrity. The concept of academic dishonesty or cheating, detailed at greater length in the Undergraduate Regulations, includes any misrepresentation of others' work as your own, such as unacknowledged paraphrasing or quoting, use of another student's material, incomplete acknowledgment of sources (including Internet sources), or submission of the same work to complete the requirements of more than one course.
You can learn more about the conventions of using sources by referring to the Yale College Writing Center's Web site. If ever you have doubts about when or how to cite, ask the course instructor, your writing tutor, or your residential college dean.