Primarily, it is to avoid allegations of plagiarism and academic misconduct. Secondarily, citing sources allows the reader to identify the sources that you relied on for your research.
It's easy: whenever you paraphrase the ideas of others, whenever you use a direct quotation, or whenever you cite a fact that isn't common knowledge, you need to cite your original sources, whether they are books, articles, or web sites.
There are two elements to APA Style citations:
The keys to successfully citing your sources in APA style are:
APA Style Tutorials & Help
The following resource provides assistance in citing sources according the Publication Manual of the American Psychological Association (APA):