It's important to cite sources you used in your research for several reasons:
Academic Honesty from MSU Conduct Guidelines for Students
The integrity of the academic process requires that credit be given where credit is due. Accordingly, it is academic misconduct to present the ideas or works of another as one's own work, or to permit another to present one's work without customary and proper acknowledgment of authorship. Students may collaborate with other students only as expressly permitted by the instructor. Students are responsible for the honest completion and representation of their work, the appropriate citation of sources and the respect and recognition of others' academic endeavors.
Many people think of plagiarism as copying another's work or borrowing someone else's original ideas. But terms like "copying" and "borrowing" can disguise the seriousness of the offense.
Plagiarism is an act of fraud. It involves both stealing someone else's work and lying about it afterward.
All of the following are considered plagiarism:
Most cases of plagiarism can be avoided, however, by citing sources. Simply acknowledging that certain material has been borrowed and providing your audience with the information necessary to find that source is usually enough to prevent plagiarism. (http://plagiarism.org/article/what-is-plagiarism)