Actus reus, or criminal action, is the essential physical element of criminal liability, while mens rea functions as the essential mental element. Actus reus ("culpable action" in Latin) is required to determine whether a crime has been committed, while mens rea ("guilty mind" in Latin) is considered in determining the severity of the criminal offense.
Actus reus must be present for a criminal conviction to exist in the Anglo-American criminal law system. Without criminal action, there can be no crime. The fundamental idea behind this is that it is wrong to punish people for their thoughts alone. Until a criminal action occurs there is no way to determine or measure the severity of a person's criminal thoughts, and it can easily be argued that criminal thoughts in and of themselves cause no harm to society. Besides, if our thoughts were the only requirement needed for criminal liability, we'd probably all have been to prison by now at least a few times.
Having said this, it's important to add that the definition of "criminal action" can be very subtle. For example, actus reus is held to occur in cases of attempt (e.g., attempted battery) simply because a series of muscular contractions occurred in the criminal that are believed to have come close to harming the victim. In some jurisdictions, people can be convicted of conspiracy simply through the action of agreeing. In many cases, even the omission of action qualifies as actus reus.
In cases of liability without fault, actus reus constitutes the whole of the criminal offense, even when mens rea is absent. However, in the conventional cases which form the vast majority of criminal law proceedings, actus reus is simply the backbone of the criminal offense, providing the specific structure and context of the criminal charges and court procedure. The substance of the trial - and the determination of criminal guilt or innocence - rests in mens rea.