In a paper, you write that World War II ended in 1945. You must cite where you found this information.
You borrow sections of a survey instrument from the internet to conduct your own research of a human population. You must cite the source of this survey.
If you reference a scholarly journal article, you should include the following information about the journal:
Why would you include citations in a research paper?
If you use a photograph taken by one of your friends in a presentation, you do not need to acknowledge your friend as the creator of the image.
If you need help avoiding plagiarism and citing sources properly, you can receive help from:
Plagiarism only applies to published materials. If someone verbally expresses an idea, you may incorporate the idea into your work without acknowledging the source.
To paraphrase from a textbook, you need only change a few words and phrases.