Many quads have a manual transmission especially sport quads. They operate just like a manual transmission on a motorcycle. Manually changing gears while on a quad will allow the rider to have more control and can help keep the engine's RPM in the best range possible to get maximum power when you have traction or less power (limited slip) when you don't.
It can also make it easier to turn or get out of a turn. When you're turning sharply you don't really want the transmission to shift because it could upset your balance.
Learning to shift gears on an ATV is a little easier than learning to shift gears on a motorcycle because you don't have to worry about keeping the quad standing up as it has 4 wheels. Everything else is about the same.
Manual transmissions require the use of a clutch, the throttle and a shift lever all at the same time. You may also need to use a brake at the same time if you are on a hill.
The brakes on these quads are the same as a motorcycle as well; the rear brake is operated by using your right foot and the front brake is operated using your right hand.
Since you have to use the throttle while taking off it will be easier to use your foot brake while taking off but that may not always be the best way depending on the situation.
Steep hills pose a very different problem and the technique you use to start on a hill will vary if you are facing up the hill or facing down the hill.
SxS often have a manual transmission as well, but they are more like a car. You operate the throttle with your right foot and the clutch with your left foot.
The shift lever is either a hand shifter on the floor like a Jeep or on more high performance SxS you might find them on the steering wheel to be operated by your hands. These are called paddle shifters and allow you to keep both hands on the steering wheel and be able to shift both up and down without letting go.
An automatic transmission does all the work for you, usually at the right time. Many Utility ATVs have an automatic transmission to allow you to focus on other things like towing, plowing, hauling etc.
Automatic transmission work very much like a car, and some of them even have a lever for either hi or lo gearing. The principal is based on centrifugal force, where the force moving away from the center of a spinning object increases as the speed of rotation increases.
When in hi gear the quad will travel at a much faster rate of speed but will not have as much power when going slow. In lo gear the max speed is greatly reduced but the amount of power at the lower speed is greatly increased, allowing you to tow or haul more.
Many youth ATVs have an automatic transmission, making it easier for the typically newer rider to concentrate more on handling the quad instead of trying to shift gears.
With all the different types of transmission available for ATVs it may seem confusing when trying to figure out what you need.
The best way to decide is to simply let the engineers at the manufacturer decide for you based on your other requirements for what type of ATV to buy.