Plagiarism is "the uncredited use (both intentional and unintentional) of somebody else's words or ideas."
—from "Avoiding Plagiarism", Purdue Online Writing Lab (OWL) (emphasis added)
You probably know that:
are all examples of plagiarism.
But did you also know that paraphrasing someone else's ideas without giving credit is also plagiarism?
Whenever you discuss someone else's ideas, their data, or the results of their research, you must cite that work. Citations are needed for quotations or paraphrases from another's work.
This statement needs a citation: "Prairie dogs increase the species richness of insect communities in grassland ecosystems (Davidson and Lightfoot 2007)."
This is an assertion that must be supported by the results of a scientific research study, and that study must be referred to with a short in-text citation (e.g., (Davidson and Lightfoot 2007)) followed by a full citation in a reference list at the end of your work:
Davidson AD, Lightfoot DC. 2007. Interactive effects of keystone rodents on the structure of desert grassland arthropod communities. Ecography 30: 515-525.
This statement does not need a citation: "The theory of evolution via the process of natural selection is one of the underpinnings of modern biology."
This is common, generally accepted knowledge, and does not need a citation.
How do you know if information you include in your work requires a citation? One simple rule applies: When in doubt, cite it!