What is the difference between the actus reus and the mens rea?

The actus reus and mens rea refer to the two components of a crime - they're both Latin terms which roughly translate into "the guilty act" and "the guilty mind" - it's much more accurate to describe the actus reus as the physical element of a crime (including state of affairs and omissions offences) , and the mens rea as the mental element or the "internal" element of a crime. If you find it difficult to identify these two components in a crime, try to find anything which relates to the mental state of the defendant (thereby identifying the mens rea) and whatever is left over will be the actus reus. As a fairly simple example the offence of murder is defined by common law as "the unlawful killing of a reasonable person in being under the King or Queen's peace with malice aforethought express or implied" - we can see that "malice aforethought express or implied" is the mens rea as "malice" is a state of mind. In an exam you would expand on 'express or implied' by mentioning that recklessness or intent to cause GBH is sufficient as the mens rea for murder. The actus reus must therefore be the elements "unlawful killing" and "under the Queen's peace".