Original: Once a bill has been formally approved by the executive (in presidential systems) or the formal head of government (in parliamentary systems) and consequently become law ("adopted"), it is called an "act", but the two terms are often used interchangeably in popular usage (or some moments of scholarly imprecision).
Simplified: Once a bill has become law, it is thereafter an act; but in popular usage the two terms are often treated as if they meant the same.