When you purchase a connecting flight, checked baggage is usually forwarded to your final destination, and will change planes when you do. When you purchase a layover flight, you will need to claim your bags when you land at your layover destination, and recheck them the next day when you continue your flight(s). If you happen to purchase an itinerary where multiple carriers are involved, you need to ask the check-in agent if your baggage will transfer on all flights.