An automatic transmission is not just a manual transmission that shifts automatically! The fundamental structure of it is completely different, but aims to achieve the same mechanical goals as a manual transmission, while making the driving experience more simple.
Automatic Transmission Gears
As pictured at the right, the structure starts with gear sets which have three sections: the sun gear at the center, the planetary gears around the sun gear, and the ring gear around the rest.
These can be locked by either the outside casing or the ring which connects all the planetary gears, and the different combonations of locks produce different gear ratios, including the reverse gear. In theory, most automatic transmissions could have multiple reverse speeds as well, if the right lock combinations were allowed.
Instead of using a clutch like a manual transmission, a torque converter is used.
Automatic transmissions can't be properly lubricated with engine oil; it uses transmission fluid, which is specifically formulated for the stresses involved.