BC stands for “Before Christ”: the number of years before Jesus was born. BC years count downward, so 500 BC is later in history than 1000 BC.
AD is short for Anno Domini, Latin for “in the year of the Lord”: the number of years since Jesus's birth. The year we live in now is an AD year.
AM is short for Anno Mundi, Latin for “in the year of the world”: the number of years from creation, when Adam was made. Adam lived in AM 1.