What kind of Law?

I once worked in a bank. In one branch, in an area where you might expect a high level of unauthorised overdrafts, our Manager achieved a very low level. I found out why - he told errant customers that it was "illegal" to write cheques that were postdated. He conveyed to his customers the impression that it was criminal - which it isn't. He justified his use of the term "illegal" by saying that s73 of the Bills of Exchange Act 1882 defined cheques as an instrument which was payable on demand. As a postdated cheque is not an order to pay on demand, but on a set date, it falls outside the legal definition of a cheque. Therefore it was NOT legal. VERY dubious reasoning.

A "law" is any rule which the Courts will enforce. In both the UK and US the two major forms are legislation and case law. Legislation is made by the legislative branch (Parliament or Congress). In Britain we use the terms "Act of Parliament" and "Statute" interchangeably. Case law arises from decisions of the courts. Both the US and UK have (not exclusively - Scotland and Louisiana have civil law roots) common Law systems - where the doctrine of precedent is both central and a key aspect of legal argument.

Law can itself be classified in a number of ways - but beware of the same words meaning very different things in different contexts. For example the term "Common Law" can mean
(1) the law of the entire land, as opposed to local law
(2) court made law, contrasted with statute law
(3) the system which puts precedent at the heart, in contrast with the civil law systems of continental Europe - where cases, though an aid to understanding what the law is, is not crucial to legal argument as it is in Common Law systems.

Civil Law too has meanings which change with context
(1) contrasted with Common Law - as a legal system
(2) contrasted with criminal law (being about relationships between individuals, and the provision of remedies rather than sanctions)