There are basically 4 types of bikes - the road bike, the mountain bike, the hybrid (combination of road & mountain) and the cruiser (what I call a beach bike).
The Road Bike is the fastest and lightest. It is designed for speed and meant to be riden only on roads. They have drop handle bars which create a more aerodynamic position on the bike that helps to make you go faster. So if you have the need for speed, this is the bike.
The Mountain Bike is designed for rough terrain. It's perfect for mountain trail rides with bumps, hills, quick turns, etc. It has thicker tires and special suspensions to cushion the jarring shocks that one might incur with rough terrain. It is a heavier bike which makes it more difficult to ride on roads for long distances.
The Cruiser (Beach Bike) has big tires, less gears and is used for easy crusing -- not speed or distance. This bike is all about COMFORT!!
We have Trek FX Hybrids. Mine is on a 20 inch frame and Nancy's in on a 17.5 inch frame.
Our Hybrid Bikes
We were "fitted" based on our height, leg length, etc. We bought the hybrids because we usually ride on roads, but really enjoy "Rails to Trails" paths which can be crushed stone or hard packed dirt as well. A road bike would not fare well on that type of surface and a mountain bike, while fine on the stone or dirt, would not fare well for just road riding. The hybrid would meet both our needs as we DO NOT have "The Need for Speed!" We also like the "straight" handle bars versus the drop handle bars (getting a little less flexible with each passing year.) Our bikes have standard 27 gears so you can conquer almost any hill without dying.
We did put "clipless" pedals on the bikes and bought shoes to go with the pedals. (Not sure why they are called clipless when they have clips that you use to anchor your shoe to the pedal???) They take a little getting use to, but help you both push and pull on each stroke. Our pedals have clips on one side and are flat on the other side. The flat side allows us to ride with regular athletic shoes when we want to ride to a destination and explore on foot. Not real easy to walk in bike shoes!!
Clip side of peddle
Nancy's Bike "Blue Streak"
Her bike is just like mine, only a little smaller and it has a NAME! Of course she does not carry a spare tube as she expects me to fix her flat or else bring the car to pick her up!!