When you kidnap people you take them away against their wishes. It is an unlawful act and it is usually, though not always, done in order to get money. You can walk into someone's house and kidnap his wife or children.
Here are a few examples.
• The gunmen kidnapped the children.
• They were planning to kidnap the millionaire's wife.
The word I understand comes from kidnabbing. The kid here refers to a baby goat. And nab, as you know, means, to steal.
The original kid nabbing referred to the stealing of kids-baby goats!
Hijacking, on the other hand, involves the taking over of a vehicle. It could be a plane, car, ship, etc. You cannot walk into someone's house and hijack him/her.
• Three men carrying pistols hijacked the plane.
• Carjacking is quite common in America.
COURTESY : The Hindu (The National News-Paper) - India