Why style guides?
When you are doing research papers, often instructors will indicate that they want you to use a particular kind of style in citing references. There are ones used for the social sciences, the humanities, and the sciences.
The most popular are American Psychological Association (APA), Modern Language Association (MLA) and Chicago. Turabian is a very useful style guide if your instructors do not indicate a particular type.
In any case, you need to cite your sources in such a manner that other can go to the sources you used in your papers. And it is very important to be consistent in your citation styles.
The following are free resources that can help you generate citations of various styles by just inputting the citation information.