Malachi, which means "My messenger", focuses largely on the corruption of the priests, the neglect of God's Temple, and the personal sins of the people. He was sent as a prophet sometime between 450 - 425 BC to Jerusalem at a time when the spiritual zeal of the people was at a low.
The Bible's book of Malachi is the last book of the twelve minor prophets, and the last book in the Old Testament. Malachi ends his book with a prophecy concerning the coming Messiah and his forerunner, who was John the Baptist (called Elijah). In this way, the Old Testament ends looking toward what God would do in the New Testament.
The theme of his book is that the people had not learned the lessons that they should have learned from the Babylonian Captivity that had taken place shortly before the time of Malachi. The people were sent into captivity as punishment for their sins, and now they were doing the same things all over again. But Malachi finds hope in the coming Messiah who will make all things right because He comes with the power of God.