Professional bicycle retailers can fit you properly to a bike, assemble it professionally, and give you the kind of advice and continuing service you need to ride safely and comfortably. Beware of retailers who do not have on-site service departments, or do not offer these kinds of services. They may not meet your needs. These are rugged bikes for off-road use, but many people ride them on pavement as well. Mountain bikes feature fat knobby tires for comfort and traction, flat bars for a more upright seating position, and low gears for easier hill climbing.
Some mountain bikes have suspension for increased shock absorption.
It depends on how and where you plan to ride. These are meant for pavement riding, and are built for speed. They have narrower tires and drop bars for a more aerodynamic position. Hybrids generally have treaded tires which are narrower than mountain bike tires, flat bars, and higher gearing than mountain bikes. They're not quite as fast as road bikes on pavement, and not quite as rugged as mountain bikes on the road.