In the first place Taser is a brand and there are other similar products on the market. Tasers products are range of self-defense devices. By doing so they can stop attackers before they get to you. Usually these devices have a reach between 5 and 25 feet. They administer shocks for five to twenty seconds depending on the model.
Hence tasers deliver a prolonged shock and pain to large muscle groups. As soon as a taser makes proper contact with an attacker he will experience pain muscle contractions and perhaps even pass out.
Stun guns on the other hand are self-defense weapons used in contact. They work when they physically contact the attacker. You have to wait until the attacker reaches you to use them since they have no range. In addition to administering pain it does not deliver a similar electric current to the taser. As a result when a stun gun makes proper contact with a person he will feel intense pain in the contact area.
A charge of a Taser will only be good for one use. If you have a spare cartridge with you you can reload bullets when you discharge those wires. Fortunately you can remove the cartridge from the taser and use it as a stun gun. Unfortunately this eliminates your range advantage so you will have to get close to your opponent.