Actus Reus is a physical component of the crime. It is a Latin term which can be translated as a guilty act. Remember that the law does not punish a person for just having bad thoughts. You may think you are committing a criminal offense. In fact, you may even decide or intend to do so, which means you have the mens rea of the criminal offense, but the law won’t punish you unless you act on your guilty mind. For example: you might decide to steal a criminal law textbook from a friend if you dishonestly intend to steal the book, you have the mens rea of the crime of theft but you will not be guilty of theft unless you commit the physical aspect of the offense i.e. doing the actus reus or the guilty act. Sometimes the physical aspect of a criminal offense is minimal, such as conspiracy to commit an offense, the actus reus for that requires agreement between two or more people to commit an offense. Despite the minimum degree of conduct involved, a physical aspect is required.
In many of the cases, the actus reus shall be something the defendant performs i.e. an act. The defendant must be in control over his/her actions. Hence, his/her actions must be voluntary. If the defendant has no control over his actions, then he has not committed an actus reus. There are very rare cases in which the accused has been convicted even if he did not act voluntarily. For example: Hill vs. Baxter, 1958, in which the accused was not in control of his vehicle due to a swarm of bees.
Another example of the Larsonneur 1933 case in which the defendant was asked to leave the UK because his visa ran out. She left and found refuge in Ireland but was sent back to the United Kingdom for being charged there. She was arrested and accused of being a foreigner to the country and sentenced.
For some of the crimes, the actus reus must also have an outcome. This can be very well seen in the offense of ABH where the act must cause some harm to the victim.
Therefore, Actus Reus is just a fancy word for a criminal act. You cannot commit a crime without action, but even if you want to commit a crime you may not actually succeed, this is called an attempt. An attempt is a special type of crime that has two basic requirements: firstly, the guilty mind or intention of committing a crime and secondly, an act something important enough that the attempter is showing that they really want to commit the crime but not actually the whole of the crime. The world is full of such type of criminals who haven’t quite succeeded due to many kinds of reasons.
The difference in the criminal act is the vital component. If you did the whole thing, it wouldn’t be attempted theft. It would just be a theft, for example, grabbing a purse, but not actually taking it. It would be an act big enough to be an attempt, but touching a purse in the process of passing, when you might think grabbing it, wouldn’t be enough, even if you thought about it.