I lump drop-bar bikes into four groups: Road Racing, Cyclocross, Sports/Touring and Loaded Touring. I look at the size of tire the bike will accept and the type of brakes installed and the length of the wheelbase.

Road Bikes have short reach caliper brakes and are usually limited to 700x25 sized tires and have a short wheelbase. Think Salsa Podio: http://salsacycles.com/bikes/podio/

Cyclocross bikes have Cantilever brakes and can accept fat tires from 700c35 or larger. Some Cyclocross bikes now have disc brakes. Think Salsa Chili Con Crosso: http://salsacycles.com/bikes/chili_con_crosso/

Sports/Touring bikes have long reach caliper brakes and can accept fatter tires, up to 700x32. Think Salsa Casseroll: http://salsacycles.com/bikes/casseroll/

Heavy touring bikes are like sports touring bikes, except that the wheelbase is longer and the bike is outfitted to easily accept front and rear racks. Heavy Touring bikes often have cantilever brakes. Think Surly Long Haul Trucker: http://www.surlybikes.com/bikes/long...cker_complete/

I have a Road Racing bike, a Cyclocross bike and a Sports/Touring bike. If I had to use just one bike for all kinds of riding, it would be the Cyclocross bike.