SR5 initially stood for Sport Rally 5, but Toyota has since stopped calling it that. It's been applied to many different Toyota models, from the Celica and Corolla to the Tundra. It is currently used on the truck and SUV line, at least in the US.
2WD and 4WD stand for the number of wheels that are powered. AWD is similar to 4WD, but AWD cars don't have the option to go to 2WD, where 4WD cars tend to have that ability. This isn't universally true on the back of cars though.
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