You put BC after a year to show that the year is before the birth of Jesus Christ, on the timeline most history books still use. 44 BC is the year Julius Caesar died. The numbers get smaller as you move toward that birth: 500 BC is later than 800 BC. This is school and history language, not slang. Though actually, the people living in 500 BC did not call their year “500 BC”; later writers stamped this label on the past. Some writers now prefer BCE for the same dates, so the world’s timeline does not have to name Christ. AD (or CE) marks years after that point. Careful: this system has no year zero — 1 BC is followed by AD 1.