• To give your writing credibility. You show that you have gathered ideas from worthwhile sources.
• To help the reader, by enabling them to check and read those sources if they wish.
• To prevent yourself from committing plagiarism. 

• All words quoted directly from another source.
• All ideas paraphrased from a source.
• All ideas or materials borrowed from another source: images, videos, artwork, statistics, graphs, algorithms, charts, etc.
• All ideas or materials taken from the Internet.

General knowledge

You do not have to cite sources for knowledge that is generally known, like the dates of famous events in history or the names of past Prime Ministers. Similarly, phrases like the "generation gap" are generally understood by the public. 

Disciplinary common knowledge

Within your field, there may be terms which are "common knowledge" because they are part of the knowledge shared by people in that field, like the "language experience approach" for educators, or the term "Impressionism" for art enthusiasts. 

When in doubt, it is prudent to cite!

When you are using someone else's exact words, you need to place quotations marks ("...") around the words. You also need to be careful not to rephrase or reorganize the words; otherwise you could be misrepresenting the author. If you want to leave out part of the author's sentence you can use three ellipsis points (...) to show the words which have been omitted.

Directly after the quotation, you should indicate where the information comes from, using one of the standard citation methods (such as MLA, APA, Chicago, IEEE, etc.) to document your sources. For more specifics, refer to the Concordia University Library's Citation and Style Guides.

Many students are unclear about paraphrasing. It is not acceptable to take the original phrasing and to rearrange a few of the original words, or to use synonyms to replace those words in order to produce a paraphrase; neither is it acceptable to use the same sentence structure, but just rephrase a few key words. When you paraphrase, it is important that the words used to express an idea are different from the original. It is your understanding of the content/text in your own words and not just the author’s text in your own words.