Criminal law is a method by which a state regulates the lives and behaviour of its citizens. A crime can be defined as an event which the law prohibits, which can be followed by a prosecution in criminal proceedings, and which gives rise to punishment of individuals, including imprisonment.
However, that definition is procedural, and tells us little about criminal offences themselves. So, the criminal law tells citizens what they must not do. Through the use of courts, judges, lawyers and juries, it puts to trial those against whom a case of a criminal offence can be made (defendants), it convicts those who are proven beyond reasonable doubt to have breached its provisions without any applicable defence, and it has at its disposal a range of punishments with which to enforce its judgments.
Certain forms of individual behaviour are regarded as so seriously wrong that almost all societies prohibit them, and attach punitive sanctions. According to Allen, behaviour is criminalised because 'it consists in wrongdoing which directly and in serious degree threatens the security or well-being of society, and because it is not safe to leave it redressable only by compensation of the party injured.' (Legal Duties,  233-4).
Others distinguish crimes from civil wrongs on the basis of morality. But the one clear unifying factor of all criminal offences is that the state has chosen to control those types of behaviour because there is a public, and a political, interest in doing so.
Study of criminal law at LLB or CPE level focused upon the most serious offences against persons and property, but it is also important to look at at the general principles, policies and theory behind all offences.