Primary sources are the law itself. These sources include: statutes (and codes), regulations, treaties, and case law.
Secondary sources provides commentary and analysis of the law. They may be written for legal practitioners, the general public, or law students. Some common examples include: treatises (such as Chisum on Patents or Wright & Miller on Real Property), hornbooks (designed for law students), legal scholarship (law journals and reviews), newsletters, jury instructions, practice materials, formbooks, encyclopedias, and etc.