An imaginary example would be wonderwidgets.com . An individual or organisation registers the domain, and uses it for two years, but at the end of those two years forgets to renew it. The domain then becomes available for others to register. Another person registers the domain, and then adds a web site to it.
The advantages of 'hijacking' an existing domain, as opposed to registering a new one, is that some sites may already be linking to it, so that the new owner will have instant backlinks and, theoretically, instant traffic.
The process is controversial, however. A recent example involved a UK-based organisation called the Poetry Society, whose domain was 'hijacked' by a company called Ultimate Search, based in Hong Kong. Although the society managed to get their domain back eventually it ended up in a lot of confusion and embarrassment for the society.
The main issue with domain hijacking is that usually the new owner will upload content that is unrelated to the site that was there previously. For example, Ultimate Search will replace any domains they take ownership with a series of 'search results' that generate revenue for its owners but do not offer the user anything useful. Others will display a 'This site is under construction' message, but include some sneaky hidden links that search engine robots may follow.
Google recently modified its search algorithm to 'reset' the PageRank of any web sites which have changed ownership, to reduce the effect of instant backlinks. Similarly, directories like the Open Directory Project now have methods of dealing with this problem without it affecting the quality of their results.