Unlike manual gearboxes, automatic transmissions don't contain individual gears. On the contrary, they're built around what are known as planetary gearsets that can be configured to produce multiple gear ratios. Instead of a friction clutch, a hydraulic torque converter is used to swap ratios. Automatic transmissions also entail the use of components such as stators, clutch packs, lockup clutches, bands, pumps, turbines, etc. Fortunately, you don't have to understand how the technology works to use it. But for the technically inclined, here are some insights to the inner workings of an automatic transmission. And this site offers additional details and some spectacular animations.