In a few of the latest cars available, you can change gears simply by pressing a button, turning a knob or toggling a small joystick. Yet simultaneously, plenty of different automobiles still require motorists to use one foot for the clutch pedal and another for the gas, all while using one hand to control the gear-shift lever through a distinct pattern of positions. And many other current cars don't possess any traditional gears at all in their transmissions.
But whether or not a vehicle includes a fancy automatic, an old-college manual or a modern-day continuously variable transmitting (CVT), each unit must do the same job: help transmit the engine's output to the generating wheels. It's a complex task that we'll try to make a little simpler today, starting with the fundamentals about why a tranny is needed in the first place.
Let's actually start with the normal internal combustion engine. As the fuel-air blend ignites in the cylinders, the pistons start upgrading and down, and 
that movement is used to spin the car's crankshaft. When the driver presses on the gas pedal, there's more fuel to burn off in the cylinders and the whole process moves faster and faster.
What the transmission does is change the ratio between how fast the Variable Speed Drive Motor engine is spinning and how fast the driving wheels are moving. A lesser gear means optimum overall performance with the wheels moving slower compared to the engine, while with a higher gear, optimum performance comes with the wheels moving quicker.
With a manual transmission, gear shifting is handled by the driver with a gear selector. Many of today's cars have five or six forward gears, but you'll discover older models with from three to six forward gears offered.
A clutch is utilized to transmit torque from a car's engine to its manual transmitting. The various gears in a manual transmission allow the car to travel at different speeds. Larger gears offer lots of torque but lower speeds, while smaller gears deliver much less torque and invite the car travel quicker.