he name says it all. Manual transmissions are the ones that you shift from one gear to the next. Automatics shift themselves, based on a series of engine load and vehicle speed signals.  All you do is put them in drive. 

The transmission is a series of gears, meshed to provide a patch for the power to travel. By moving the shifter, the slider locks one of the gear sets to the output shaft, providing a specific path for power flow.  Each gear set provides a different gear ratio, so by shifting through the gears, you can choose which gear range suits the driving conditions. 

Automatic transmissions also provide different gearing, but they use oil pressure to apply individual clutch packs or bands inside the transmission to create the various gear ranges.  By applying a clutch pack or bans, the clutches grab onto one of the gear train components, which either holds or turns that component.  The combination of holding and turning enables the gear set to create different gear ratios within the transmission.  

What really makes the automatic transmission different is that it shifts automatically.  By measuring engine load and road speed electronically, a computer system controls transmission operation.  The computer decides when the transmission should shift and then delivers an electrical signal to create the shift.  The transmission still uses oil pressure to control the clutches, but now the computer regulates which clutches apply and when.  

Another difference between automatic and manual transmission is how they transfer power from the engine.  Manual transmissions use a clutch; automatics use a fluid coupling called a torque converter. 

A torque converter is an oil-filled device that uses oil flow to transfer power.  An impeller mounted to the crankshaft sling the oil into a turbine, which connected to the transmission input shaft.  This oil movement transfers the motion from the crankshaft to the input shaft without the two being connected mechanically.  That’s why you can come to a complete stop in a car with an automatic transmission, without having to press a clutch pedal.  There’s no mechanical connection that has to separate. 

Once the oil gets through the turbine, it’s redirected through another set of fins called a stator.  The stator prevents the “used” oil from dragging against the torque converter, so it increases the torque converters’ efficiency.  

While very efficient, there is some power loss through the torque con­verter. What's more, during normal operation, torque converters create a lot of heat. To eliminate that heat and pick up a little extra efficiency, most manufacturers now add a torque converter clutch to the converter. This clutch applies once the car is cruising down the road to create a direct mechanical connection between the engine and transmission, eliminating the slip and reducing operating temperatures.

While most cars use one of these types of transmissions, there are a few less-common types. Some early vehicles used semi-automatic transmis­sions, which still required manual shifting but didn't have a clutch pedal. Some of these transmissions were based on a manual transmission, using vacuum and electric controls to operate the clutch. Others were closer to automatics, but without the governor to force the shift automatically. 

The CVT, or continuously variable transmission, uses a special belt that runs between two pulleys to transmit power between the engine and the wheels. As the power and speed requirements change, the pulleys change in width, which alters how deeply the belt rides in them. This changes the effective radius of the pulley, which varies the "gear ratio." 

Finally, whether you buy a car with a manual or automatic transmission is really a matter of preference – there’s no good choice or bad choice.  It won’t be long before we won’t have to worry about transmissions at all. Some of the new electric or hybrid gas/electric cars don't use any transmission at all. The primary motor on these cars is electric, and it operates under a much wider range of speeds, so a transmission isn't necessary!

Also see

Your Car's Mechanical Condition

7 Easy Steps to Make Your Clutch Last Longer -some of these may surprise you.

What is a Differential? -During a turn, the outer wheels drive farther than the inner wheels, and this is an important function of the differential.

Dealing with Transmission Problems -learn about some common transmission problems for manual and automatic transmissions.

Maintaining the Suspension and Steering -relatively dependable and trouble-free; however, there are a few things you should do to keep both in good working order.

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