4x4 auto: you have a transfer case with a center differential, allowing the front axle to turn at different speed than the rear axle. Use this setting for driving in winter, on sand, and anywhere traction is lower than nice pavement
4x4 lock: on trucks with a center diff, the center diff if locked. On trucks that don't have "auto", it simply engages the front axle. Front and rear axles must turn the same speed. Use only on low-traction surfaces, such as ice, snow, sand, offroad, etc. Can run at highway speed if you're on heavy snow.
4x4 low: locks both front and rear axles, and uses the transfer case low range. Use only on low-traction surfaces where maximum torque is required (like pulling out a truck from the mud, yanking a tree stump).
