It is very important to cite your sources for two reasons: 1) to avoid the appearance of plagiarism (claiming the words or ideas of someone else) and; 2) to allow the reader to find the source themselves. This means that URLs by themselves are not sufficient, because they can change, and may not include enough information to identify the work.
At the very least, a citation should contain a title, an author (if applicable), and a date (if available).
The two most common citation styles in the social sciences are:
Legal citations enable a reader to easily locate the cases, statutes, regulations or other material cited in legal documents. They often appear as a number, an abbreviation of the publication in which the document is found followed by another number, e.g. 549 U.S. 497 (2007) is the citation to Massachusetts v. Environmental Protection Agency. This case can be found in volume 549 at page 497 in the United States Reports.