We do not know who Malachi was. Malachi might have been his birth name, or perhaps his nickname, as God's messenger. He was active during the reign of Persia, around 500 BCE.
The subjects with which Malachi was preoccupied were different from those that preoccupied the classical prophets and reflected his time. But one can find in them beautiful comparisons to today's problems as well. One of the main subjects in his prophecy was the covenant between God and Israel, as well as his criticism of the people's disdain for the worship of the temple, and of the priests for misusing their position.