What causes engine to shake

You need to provide a whole lot more details. Misfires can be called "shaking". Collapsed engine mounts transfer normal engine vibration into the passenger compartment. How long since the last tune-up? Does this happen all the time? Just when accelerating? Only at idle? Only when first started? Do you feel it in the steering wheel, seat, or entire vehicle? Do you hear a misfire at the tail pipe? Is the Check Engine light on? Did you recently buy gas? Don't make us guess.