I encourage my students to work together, both during most class activities and in study groups outside of class.  We call this type of work cooperative learning.  One of the keys to cooperative learning is that everyone is an active participant.  Each person shares their own thoughts and ideas, and everyone benefits from that sharing.

In contrast, plagiarism is defined as "the appropriation or imitation of the language, ideas, and thoughts of another author, and representation of them as one's original work."

The difference between cooperative learning and plagiarism:
One semester I had two students who obviously worked together.  They came to class together and always sat next to each other.  I could also see from grading their homework that they worked together, but what I saw was not the product of cooperative learning.  It was obvious that one person was doing the work and the other person was copying that work.  When one person made a mistake, even a simple arithmetic error, the other person reproduced the identical error.  Toward the end of the semester, one of the students admitted to me that he was doing the homework himself.  Each morning he would go to the cafeteria for breakfast before class, and his friend would copy his homework.  He knew this was wrong, and tried to encourage his friend to either work with him or do his own homework, but his friend refused.

I expect academic honesty from all of my students!  
   Whenever you are taking a test or quiz, whether in class or in the testing center, I expect you to work alone, and show me the mathematics that you can do without the use of books, notes, conversations, or any other source of outside assistance.
   Whenever you or another student has taken already a test or quiz, do not discuss or share information about that test or quiz until after everyone has taken it.
   Whenever you are preparing a homework assignment or project that is not a test or quiz, I encourage you to work with other people to learn the material and correct mistakes, but always turn in your own work.  Copying another person's work or permitting another person to copy your work is not academically honest!

If you are caught participating in any act of academic dishonesty (i.e. cheating or plagiarism):
   All participants in the incident will receive a grade of zero (0) for that assignment, quiz, or test.
   All participants in the incident will be reported in writing to the Vice President of Student Services.  
   That report will become a part of your permanent academic record.
   You will be subject to whatever additional disciplinary action is deemed appropriate by the Vice President, in accordance with the Student Code of Conduct.
   The punishment for multiple reported acts of academic dishonesty is expulsion from the College.