Automatic Transmission (AT)
This is a transmission that uses a torque converter, planetary gearset and clutches or bands to shift through a vehicle's forward gears automatically. Some automatics allow the driver a limited amount of manual control over the vehicle (aside from choosing a forward, reverse or neutral mode) -- for example allowing the driver to control upshifts and downshifts by utilizing buttons or paddles on the steering wheel or the gear selector. Common names for such transmissions are "shiftable automatic," "Tiptronic" and "autostick." To learn more, see Automatic Transmissions: What Makes Them Work.
Manual Transmission (MT)
With a manual transmission, the driver selects all gears manually using both a movable gear selector and a driver-operated clutch. This type of transmission is also known as a "stick shift" or a "standard" transmission. See Manual Transmission Basics for more information.
Automated Manual Transmission (AM)
Like a manual transmission, an automated manual also employs a mechanical clutch; however, the action of the clutch is not controlled by the driver via the clutch pedal but rather is automated using electronic, pneumatic or hydraulic controls. Sometimes referred to as a "Direct Shift Gearbox" ("DSG") or a "Sequential Manual Gearbox" ("SMG"), this transmission allows for either fully automatic forward gear shifts or manual shifts through the gear selector or through buttons or paddles on the steering wheel.
Continuously Variable Transmission (CVT)
This transmission has a continuously variable drive ratio (as opposed to conventionally stepped gear ratios) and uses belts, pulleys and sensors rather than gears to maintain a steady acceleration curve with no pauses for gear changes. Because of this, a CVT can keep the engine in its optimum power range, thereby increasing efficiency and gas mileage. You can get more information from CVT Enters the Mainstream.