Citing is how you give credit to the sources you have used in your paper and avoid plagiarism. It allows other scholars (and your professor) to find those sources and verify your research.
When writing a paper, you MUST identify the source of any
A Citation style is a collection of guidelines used to make sure written papers follow a clear and consistent format. Some common elements of citation style are: headings, punctuation, presentation of numbers and statistics, construction of tables and figures and citation of references (Source: Official APA Website)
Most disciplines have a specific citation style that they follow. If you are not sure, consult with your instructor to determine what citation style is required for your particular course.