They both can increase the amount of fuel in the fuel/air ratio going into the engine, but the mixture control does it by indroducing more fuel, while the choke does it by decreasing the amount of air.
Cars also have a mixture control, but it is set by screw driver on the carburretor itself, under the bonnett. As cars usually stay at roughly the same height above sea level, it isn't necessary for them to adjust the mixture while driving.
(From memory!
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