Whether you quote, paraphrase or summarize, you must always provide a citation for the sources that you used.
Information considered to be "common knowledge" does not need to be cited (unless you are directly quoting an author). If you are unsure, err on the side of caution and cite the source. "Common knowledge" refers to anything that you expect the readers to already know. An example of general common knowledge is that George Washington was the first U.S. president.
Common knowledge can also be field-specific. For example, a nursing student would not have to cite a basic definition of systolic and diastolic blood pressure, since students and instructors in that field are familiar with the concept already. Here are a few more examples:
- The sky is blue.
- The physics of light refraction explains why the sky appears blue.
- Ernest Hemingway wrote "The Sun Also Rises".
- Hemingway's life experiences greatly impacted the story, characters and setting in "The Sun Also Rises".
Paraphrasing is taking a small excerpt from your source and putting it into your own words, whereas summarizing gives an overview of the main points from an entire source (e.g., and entire book or article). In both cases, it is important to cite your sources. The video below explains more. For more detail and examples, see Purdue OWL: Quoting, Paraphrasing, and Summarizing.
Purdue OWL provides an excellent example showing quoting, paraphrasing and summarizing in the same paragraph:
In his famous and influential work the Interpretation of Dreams, Sigmund Freud argues that dreams are the "royal road to the unconscious" (page #), expressing in coded imagery the dreamer's unfulfilled wishes through a process known as the "dream-work" (page #). According to Freud, actual but unacceptable desires are censored internally and subjected to coding through layers of condensation and displacement before emerging in a kind of rebus puzzle in the dream itself (page #).
Quoting: "royal road to the unconscious" and "dream-work"
Paraphrasing: "According to Freud, actual..."
Summarizing: "In his famous and influential work..."
For more paraphrasing examples, see Successful vs. Unsuccessful Paraphrasing.
Source: Driscoll, Dana and Allen Brizee. "Quoting, Paraphrasing, and Summarizing." Purdue Online Writing Lab. Purdue University, 14 Dec. 2011. Web. 27 Jul. 2012.