What is a thriller?

Thrillers are mainly fast paced and tension filled. They often take place in exotic settings such as foreign cities or harsh environments such as the dessert. The heroes in most thrillers are frequently "hard men" and are very stereo typical of spies or hitmen. However, sometimes they may also be ordinary people who become involvedin the danger by chance. While the protagonist is mainly a male female lead characters have become increasingly common; an example would be sigourney weaver's character 'Ripley', in the movie 'Alien' (1979).
Thrillers often overlap with mystery stories, but are distinguished by the structure of their plots. In a thriller, the hero must prevent the plans of an enemy, rather than uncover a crime that has already happened. Thrillers also occur on a much bigger scale: the crimes that must be prevented are serial or mass murder, terrorism, assassination, or the overthrow of governments. While mysterys end with the solving of the mystery, thrillers usually end in the hero defeating the villain. In short if it thrills it is considered a 'thiller'

http://en.wikipedia.org/wiki/Thriller_(genre)