Two sides of the same coin... That's how we can understand plagiarism and citation. Citing our sources is one major way to avoid plagiarizing.
Citation is a formal way of indicating in our writing, our presentations, and speeches the sources we consulted. Citing sources is called attribution: giving credit to the person or people whose ideas we are referring to in our work.
Plagiarizing happens when we fail to cite our sources.
For more info on understanding what plagiarism is and how to avoid it, consult: Plagiarism Resource Center