FayeDunaway offered a good definition from dictionary.com. A radical fundamentalist is someone who has an extreme interpretation of, for instance, the Holy Book, and seeks change or upheaval. Muslims are more likely to be portrayed as radical fundamentalists because not only do they have an extreme interpretation of Quran (when we're talking about people like ISIS sympathizers—not all Muslims), but they also seek change or upheaval in the society.
Regular fundamentalists, such as Jehovah's Witnesses (I would certainly classify them as such), simply adhere to their doctrine in a very strict way. However, they usually do not seek change in the society—with the exception of fundamentalists who don't want their relative country to accept homosexuality etc. Nonetheless, even those fundamentalists will not break the laws of the land in order to force the change upon the society, which is unlike radical fundamentalists who don't mind upheaval.
Basically, seeking upheaval in the society is what differentiates radical fundamentalists from regular fundamentalists.
