The fifty American states are separate sovereigns with their own state constitutions and state governments. They retain plenary power to make laws covering anything not preempted by the federal Constitution, federal statutes, or international treaties ratified by the federal Senate.
Nearly all states started with the same British common law base, with the notable exception of Louisiana; Louisiana law has always been strongly influenced by the French Napoleonic Code. The passage of time has resulted in enormous diversity in the laws of the states. State courts have expanded the old common law rules in different directions (through their traditional power to make law under the doctrine of stare decisis), and state legislatures have passed various statutes expanding or overriding many judge-made rules.
Unlike other common law jurisdictions, all American states have codified some or all of their statutory law into legal codes. Codification was an idea borrowed from the civil law through the efforts of American lawyer David Dudley Field. New York's codes are known as "Laws." California and Texas simply call them "Codes." Most other states use terms such as "Revised Statutes" or "Compiled Statutes" for their codes. California, New York, and Texas have separate subject-specific codes, while all other states and the federal government use a single code divided into numbered titles.
In some states, codification is often treated as a mere restatement of the common law. Judges are free to liberally interpret the codes unless and until their interpretations are specifically overridden by the legislature. In other states, there is a tradition of strict adherence to the plain text of the codes.
The advantage of codification is that once the state legislature becomes accustomed to writing new laws as amendments to an existing code, the code will usually reflect democratic sentiment as to what the current law is (though the entire state of the law must always be ascertained by reviewing case law to determine how judges have interpreted a particular codified statute).
In contrast, in jurisdictions with uncodified statutes, like the United Kingdom, determining what the law is can be a more difficult process. One has to trace back to the earliest relevant Act of Parliament, and then identify all later Acts which amended the earlier Act, or which directly overrode it. For example, when the UK decided to create a Supreme Court of the United Kingdom, lawmakers had to identify every single Act referring to the House of Lords that was still good law, and then amend all of those laws to refer to the Supreme Court.
In the arena of criminal law, all states have somewhat similar laws in regard to "higher crimes" (or felonies), such as murder and rape, although penalties for these crimes may vary from state to state.
For public welfare offenses where the state is punishing merely risky (as opposed to injurious) behavior, there is significant diversity across the various states. For example, punishments for drunk driving varied greatly prior to 1990. State laws dealing with drug crimes still vary widely, with some states treating possession of small amounts of drugs as a misdemeanor offense or as a medical issue and others categorizing the same offense as a serious felony.
United States tort law varies widely across the states. For example, a few jurisdictions allow actions for negligent infliction of emotional distress even in the absence of physical injury to the plaintiff, but most do not. For any particular tort, states differ on the causes of action, types and scope of remedies, statutes of limitations, and the amount of specificity with which one must plead the cause. With practically any aspect of tort law, there is a "majority rule" adhered to by most states, and one or more "minority rules."
Attempts at "uniform" laws
Efforts by various organizations to create "uniform" state laws have been only partially successful. The two leading organizations are the American Law Institute (ALI) and the National Conference of Commissioners on Uniform State Laws (NCCUSL). The most successful and influential uniform laws are the Uniform Commercial Code (a joint ALI-NCCUSL project) and the Model Penal Code (from ALI).
Apart from model codes, the American Law Institute has also created Restatements of the Law which are widely used by lawyers and judges to simplify the task of summarizing the current status of the common law. Instead of listing long, tedious citations of old cases (in order to invoke the long-established principles contained in those cases), they can simply cite a Restatement section to refer to a particular common law principle.