Legal research categories can be determined on two different categories: the nature of the research, and the nature of the law. Nature of law specifically refers to the categories of law that one is trying to research, and are as varied as the categories of law itself.
The nature of research will generally pertain to the three categories of legal materials. (save international, which can be a much wider gray area). Evidence represents material or testimonial data that will be analyzed through legal interpretations (which are determined by primary and secondary resources).
Some particular areas of legal research can involve particular categories of material. Since most opinions are rendered in cases that involve a decision rendered by a judicial vote from a judiciary, they can take the form of majority opinion and minority opinion). Ordinances refer to laws that are mandated by an authoritative body, while executive orders are laws that are implemented by an executive branch of government. Court rules refer to specific rules of decorum and procedure that are required by specific courts and committees.
Otherwise, the other format of legal research categories are identical to the categories of law itself, but a general overview follows:
Criminal law refers to acts that violate the statutes by which social behavior is governed, where violations of these statutes (crimes) are considered actions against the state and will be pursued and punished by the government. Tort law covers negligence where damage is accrued to an individual’s person or property, which can have numerous subcategories, including medical malpractice and personal injury.
Administrative law refers to the administrative decision and regulations that are enacted as law on the behalf of government agencies, while Constitutional law involves constitutional amendments, which serve to instruct government and civil rights throughout various levels of society. Most legal systems are divided between civil law, where laws are legislated, and common law, which is crafted through judicial decisions that establish unwritten precedents (the United States is a civil law system that incorporates common law oversight in the form of judicial review).
Often a researcher will be tasked, on the basis of the work of their firm, with research laws related to that category of law, but since many legal actions cross these legal categorical boundaries, it is not uncommon for a researcher to perform research outside of their realm of experience.