Any words, ideas or images that you do not create yourself must be properly credited to avoid plagiarism. Citing information sources acknowledges the origin of your information and it provides support and credibility to your work by showing evidence of your research. A citation is a reference to the source of an idea, information or image. It typically includes enough identifying information, such as the author, title, date, publication format, etc.
Quick citing tools can help you get started, but ALWAYS check the Style Guides for specifics as you are responsible for accurately citing your sources.