These tools help manage, store and organize references that could be used in papers, dissertations, and theses. These are more popular citation programs. There is a good comparison chart by Wikipedia -- Reference Management Software.
A citation is information needed to locate the reference (article, book, website or other) you are interested in reading. Citations are also used to help readers of your papers go to the information you used. The citations are found on the References page (APA style) at the end of a research/term paper.
These links can assist with creating your references for your papers.
REMEMBER!!! These only assist with writing citations. They are not perfect and you still need to know the rules for the style assigned (APA, MLA or others).