A hijacker is a person who takes control of some craft or object by
force. A three common
examples of this is the people who steal from truckers, people who change a website or steal information from it, and airplane hijackers.
Truck hijackers are some of the common thugs in theft, when a large truck is taken, the merchandise would be sold to fences or other forms of distributors. Some mobsters had started their life crime as truck hijackers and the practice is still in use to some extent.
Web site hijackers will tend to take over a website for gain in popularity, information, or monetary units. Website hijackers tend to steal information from websites through sniffers or other means, even breaking in. They can also be ones who steal media, content, or redirect the sites. There is no formal definition for the Internet as a cut and paste could be considered a hijacking in many cases.
Airplane hijackers were extremely numerous in the early 90s. Quite often on the news a story of another commercial plane would be taken and either ransomed or taken to Cuba or another third world country. After 9/11 the fear of airplane hijackers have resurfaced.