One of the basic principles of scientific writing is to cite the sources. Therefore, you have both in-text citations and reference lists in your document.
In-text citations guide you to the reference list, where you can find more specific information about used sources.
Write all used sources to the reference list, in order to:
Cite primary source which reports original content. Cite secondary source only when the original work is unavailable or is in a language you do not understand. In the text, identify the primary source. Only the secondary source appears in the reference list. See the APA guideline.
Citation styles vary by disciplines and by publications: