The frame will be heavier to better handle a load without going noodly. The geometry will be what s called more relaxed, for a softer and more comfortable ride. Seating typically is a bit more upright. The frame will have attachment points for as many as three water bottles, plus front and rear racks and fenders. Straight handlebars, squat frame, 26-inch wheels with fat tires, these days almost certainly a front suspension fork.
Mountain bikes are comfortable and easy to ride, and adept at hopping from road to trail to path.
The frame and wheels themselves offer plenty of shock absorption, so you can forego the expense and weight of a front shock. Now, if you really intend to cover some miles on roads, but tour only occasionally, look to a category called sport bikes.