APA style includes an in-text citation in your document to briefly identify the source you have quoted or paraphrased and a reference list for all in-text citations at the end of the document.
Citing sources correctly is important to:

• give credit to the person who came up with the original idea
• direct readers to the original source for more information
• prove that you're basing your work on solid research
• avoid plagiarism

For a general overview of APA referencing and practice examples see the tutorial on APA referencing.