Another definition, a newer one, is the following one (the details can be found here):
Hijacking is a type of network security attack in which the attacker takes control of a communication - just as an airplane hijacker takes control of a flight - between two entities and masquerades as one of them. In one type of hijacking (also known as a man in the middle attack), the perpetrator takes control of an established connection while it is in progress.
Pagejacking is the process of copying the content of a web page by somebody (some third party) who thinks the text is interesting and he wants it for himself/herself. Usually this somebody have not been authorized to do so and he/she rarely put a backlink to the original content to his/her personal page. As a result a part of the traffic that should have gone to the original page ends up to the new page, and that's why they do this.
Pagejacking is another word for theft, the people that do this create duplicate content and lately the search engines (like Google) changed their algorithm to put accent on the original content and to penalize things like this. On short terms the pagejacking might help improve the traffic on the targeted webpage, but on the long term there will come no good things from this.
How is pagejacking done?
The simplest and the most common way is to copy/paste the content, including the html tags, to the targeted site. In some cases the html tags are stripped, some links are added, changed or deleted, some words are also added or changed to appear as unique content.
Sometimes, when there are involved more experienced pagejackers, they use advanced techniques as serverside coding languages (php, jsp, asp and others) to detect the search engines spiders landed on their web site. To the crawlers they show the duplicate content and to the human visitors they show other content, like the products lists they want to sell.
How do I prevent pagejacking?
There is no way to prevent people from copying your content: every time a visitor comes to your site the content of the webpage is sent to their computer. They can always get it. You can not prevent it, you can only monitor who comes to your site and you can correlate the visitors to the duplicate content you (may) find on the internet.
Why unique content?
Because it is the thing that makes your web page attractive. The design will make your web site shine, but it's unique content will make it distinct. They will make the visitors (both humans and spiders) to come over and over again and will increase your site ranking.
Why content writing?
The content writing is used to improve your page additionally to the products and/or services you want to sell. Stealing other people's content will not make your site unique, will not help your page grow in the right way. The best way to do that is to start writing your own unique content, if you can't or you don't know exactly how hire somebody that can and know that.