Criminal law is the body of law relating to criminal offences. A criminal offence is an action that is prohibited by the law. But it must be distinguished from civil offences, such as tort or breach of contract. A criminal offence is a public matter--the offence is deemed to be committed not only against the victim, but against society as a whole; and a public official (as opposed to the victim) is responsible for taking legal action against the person who committed the offence. Moreover, a criminal offence does not require a victim, nor is the consent of the victim needed for prosecution: a person can be prosecuted and convicted for a crime even if the victim is opposed to the prosecution.
This distinction between civil and criminal law is universal in developed legal systems, though many ancient legal systems did not clearly draw it: murder for instance, was often a matter between the murderer and the victim's family. But even they provided for offences without any victim, such as violations of religious or sexual taboos (e.g. incest or bestiality), so even they recognized the distinction to some extent.
The exact acts prohibited varies from jurisdiction to jurisdiction; nonetheless there is a common core of offences (e.g. murder, theft) criminalized everywhere, though the details may vary.
Sometimes a distinction is made between crimes mala in se and crimes mala prohbita. A crime malum in se is an act that is wrong in itself (e.g. murder), while a crime malum prohibitum is a crime that is only wrong because the law prohibits (e.g. not wearing one's seatbelt).
Crimes can be divided into several (overlapping) categories: crimes against persons, crimes against property, crimes against state security, sexual offences, computer-related offences.
Crimes against state security are offences such as espionage, treason and releasing information essential to national security. (Many non-democratic or semi-democratic countries use this term as a catch-all for crushing dissent.)
Computer related offences include such offences as computer hacking, overriding copyright protections on music, etc., electronic distribution of child pornography. See Final Draft Convention on Cybercrime.
See also age of consent