solaris453 has given a complete and accurate answer.
The Taser and stun gun are two completely different things. The Taser physically locks your whole body up. You cannot move when the Taser is shocking you. A stun gun causes pain, but you do not lose any ability to move. The Taser can also be used as a stun gun, if there is no cartridge in it, or if the darts have been fired and one of them misses. When it is used as a stun gun it is called a drive stun.
When you are stunned with the drive stun method you WILL instinctively jerk away from it because of the pain.
When you are shot with the electrodes you cannot move. You just lay there waiting for what seems like an eternity for the shock to stop.
And it is not simply a shock. It pulses 19 times per second, as solaris stated. When you are tased you feel the constant flow of electricity through you. You also feel those pulses each time they hit. It feels as if you are being shocked constantly, and punched very hard every time a pulse hits you. Add that to the psychological effect of not being able to move at all, and it is not a very pleasant experience,
I have been through Taser training in the police academy, and I know first hand what is is like to be both stunned by the drive stun method, and the actual Taser wires. Believe me, it is the longest five seconds you will ever have to go through.
Another note that may or may not be of interest.
Power is Amps multiplied by volts. (P=IE)
P = power, measured in Watts
I = current, measured in Amps
E = Electromotive force, measured in Volts
I learned this as a radio maintenance technician in the U.S. Air Force