Kidnapping is to abduct or steal a person by force without their consent and proceed to hold them hostage. Generally, the kidnapper demands some sort of ransom for the person they have kidnapped. Most of the time, with a kidnapping, the kidnapper is looking for money or a monetary payoff. Sometimes they will tie the person to a chair and tape over their mouth, as this is often seen in action movies.
On the other hand, hijacking someone means to take with conveyance by force. A person or group of people takes control of the vehicle. Conveyance refers to a car or other ways of transportation, such as an airplane, car, truck, or boat. Criminals hijack vehicles to use it for their purposes, rather than the purpose for which it was meant.