Not very much. Many bagger motorcycles are just adaptations of cruisers.  Baggers will largely be identified by the standard saddlebags that come with the bikes, and more luxury features such as premium forks/suspension, more comfortable seats, floorboards over pegs and some sort of the windscreen or fairings.  The best way to think about it is that a cruiser is a daytrip bike whereas a bagger is ready for an overnight stay or longer road trip.