According to the UAA Dean of Students, “Academic integrity is a basic principle, which requires that students take credit only for ideas and efforts that are their own.” As stated in the Academic Integrity Tutorial, it is important because "We rely on one another to live up to the highest standards because the quality and value of our own academic work depends on the trustworthiness of the work done by our colleagues. The reputation of our academic work depends on whether others can use it with confidence and know that their own contributions will be acknowledged. Similarly, the reputation of our University and respect for the value of the qualifications we gain here are earned and maintained through the integrity of our work."
Information that is considered common knowledge does not require a citation in your work. But how do you know if something is common knowledge or not? Visit the MIT Academic Integrity page for a useful explanation of common knowledge.