Your first answer is probably "to avoid plagiarism." That is one very important reason to credit the sources from which you found data or quotations used in your paper, project, or presentation.
However, it's not the only reason or even the most important. When you cite sources, you are actually participating in an ongoing conversation between scholars. You are indicating to your reader that your work is in response to the work of others, and giving them a path to retrace that conversation. You show the evidence of your thinking, analysis, and synthesis of information when you can point to the sources that helped you make your point. This is how scientific discoveries are made, and how social change occurs. It is very important!
Are there other reasons to cite sources? Add your response in the comments!
All proper citations, no matter the style, have certain key pieces.
Depending on the type of source, there might be other information to include. For example, you might have an article from a journal; then, you would need to include not only the article title but also the title of the journal. If it's a digital source, you may need to include a URL, but a URL by itself is not a citation.
This is information about the source that will help your reader evaluate the quality of your research as well as find any of your sources that they might like to look at themselves. Think about this; just as you might have chased down a few citations that you found in other sources' cited works, YOUR reader could do the same thing after reading your work. Then, they might respond to your paper and your sources with their own thesis or hypothesis. Remember how this is all one big conversation?
The style you are following gives you the rules and standards about how to present this information, both as a list at the end of your work, and as references within the body of your work.
You must cite your sources whenever you have taken specific data, information, or quotations from them. That means using a cue in the text of your writing to indicate when another's ideas or data are being used. When MLA is used, this usually means quoting another author's work to help prove your point. In Chicago, writers are often analyzing and synthesising information from primary sources. In APA, writers and readers are mostly concerned with the currency of the information they access.
This determines whether you use parenthetical citations or footnotes or endnotes.
MLA, APA, What do all these letters mean?
You have probably been told by your teacher that your paper and citations should be in MLA format (or maybe Chicago, or APA). These are citation styles. Citation styles are a set of standards or rules to follow when presenting information and using sources. Different academic disciplines generally follow a particular style.
Note: These different styles are really important to be aware of when you get to college. Sometimes at the middle- or upper- school level, teachers decide to just stick with MLA, which is usually the first style that students learn about. But pay attention to your teacher's instructions!
When it comes to reviewing a list of sources, scholars in different disciplines are looking for different things. For example, in the humanities, the author of a source is a really big deal. In History, the work itself is important. In the sciences, readers are very interested in how current the data is. So, when it comes to deciding which style is appropriate, think about what would be most important to readers in that discipline.
Here is an example of the same article cited in MLA, Chicago, and APA format:
MLA
Fuller, Charles F., Jr. "Edwin and John Wilkes Booth. Actors at the Old Marshall
Theatre in Richmond." Virginia Magazine of History and Biography 79.4
(1971): 477-83. JSTOR. Web. 12 Feb. 2015. <http://www.jstor.org/stable/
4247681>
Chicago
Fuller, Charles F., Jr. "Edwin and John Wilkes Booth. Actors at the Old Marshall
Theatre in Richmond." Virginia Magazine of History and Biography 79, no. 4
(1971): 477-83. http://www.jstor.org/stable/4247681.
APA
Fuller, Charles F., Jr. (1971). Edwin and John Wilkes Booth. Actors at the Old
Marshall Theatre in Richmond. Virginia Magazine of History and Biography
79(4), 477-83. http://www.jstor.org/stable/4247681.
What differences do you see? How can you explain them?