"An Extreme Tourist is someone who explores, experiences and totally immerses themselves in their travel destination - mind, body and soul." It's a definition provided by Extreme Tourist the independent traveller's web community devoted to active travel, adventure and eco tourism.

The Wikipedia ads that extreme tourism - that is also known as shock tourism - is a type of niche tourism involving travel to dangerous places (mountains, jungles, deserts, caves, etc.) or participation in dangerous events. As we can see from those two definitions, extreme tourism overlaps with extreme sport because it they share the adrenaline rush caused by an element of risk.

Interestingly, Word Spy indicates hat the second definition was created by "Dennis Tito, the poster boy for "out-there" vacations. (In case you forget, Tito was the businessman who paid US$20 million to the Russian Aerospace Agency to let him hang out at the International Space Station for a few days.)". 

Extreme tourism is a growing business in the countries of the former Soviet Union - such as Russia, Ukraine, Azerbaijan, etc. and in South American countries (e.g., Peru, Chile and Argentina). The mountainous and rugged terrain of Northern Pakistan has also developed into a popular extreme tourism location, but because of the Al Qaida scare it's not as popular.