Primary sources have documents that are "the law." The documents are created by bodies with legal authority. For example, cases are decided by courts; statutes are enacted by legislatures; regulations are issued by administrative agencies.
Secondary sources are all the materials that are about the law. They are written by lawyers, law professors, law students, independent authors, and publishers' staff writers. They are useful in understanding the law, but they are not the law. You can't go to jail for disobeying a treatise or failing to comply with a dictionary.
Law has many types of secondary sources, used for different purposes:
A law library also includes many of the sort of "regular books" you might see in a bookstore or public library, such as works of journalism, history, or political commentary.
Discussions of legal issues are also found in blogs, law firm and government websites, and even Twitter, and so these newer media are secondary sources as well (although this guide doesn't cover them).
Photo credit: U.S. Supreme Court building from http://www.supremecourt.gov/.
It is often useful to start your research by finding an on-point secondary source, especially for an unfamiliar area of law. But why? Why not just start searching for relevant cases or statutes? Well, as explained by Kent C. Olson in Legal Research in a Nutshell (13th ed. p. 25),
Primary sources such as statutes and cases can be confusing, ambiguously worded documents. Secondary sources are usually more straightforward and try to explain the law. They summarize the basic rules and the leading authorities and place them in context, allowing you to select the most promising primary sources to pursue.
In other words, starting with a secondary source is more efficient and will provide you with an understanding of the forest AND help you identify the most important individual trees.
If you would like additional instruction on secondary sources, check out this lesson on CALI (visit the law student restricted databases page if you need the CALI authorization code):