Lower KV motors have more torque
Higher KV motors have less torque but more top speed

Multiply the KV by the voltage given to the motor

ie - 3300 x 7.4v(2s lipo) = 24,420rpm
6600 x 7.4v = 48,840rpm

Lower kv motors are typically used for lower speed/smaller tracks, heavier cars/trucks and typically 4wd models.

Hope this helps