Fanaticism is defined as a life-consuming obsession to some idea or cause. Fanaticism with respect to social ideals is roughly a synonym for extremism, although "fanatic" originally referred to extreme religious devotion and the term still sees more application to religious extremists.
The concept of the fanatic is similar to that of the crank, though they are not identical. The difference is that the subject of the fanatic's devotion may be mainstream even if the devotion itself is not; hence, one speaks of a fanatical Christian or Muslim, but a crank adherent to New Age ideas.
Fanatics' adherence to mainstream ideas makes them much more capable of doing damage, as they have ready access to, and influence over, a large body of non-fanatics who agree with their ideas. Notable examples are the Religious Right in the United States and Muslim fanatics such as Osama bin Laden inducing Muslims to go on suicide-bombing missions.
In the language of the Third Reich, "fanatical" was given positive connotations and was the adjective attached to the ideal Nazi devotee.