The Fundamentalist and Extremist are commonly known to be more expressive about the emotional reaction of a speaker rather than the subject itself. It may not be specific to religion as there are fundamentalists and extremists found in any religion.
The beliefs of a Fundamentalist are usually the literal interpretations of a doctrine; an example is the belief of pigs are dirty and evil thus Fundamentalist will not eat pork. They will not accept any modifications or reinterpretations to this doctrine because they believe what is written. An extremist, on the other hand, engages or supports any militant behavior that enforces their chosen doctrine. An example is the belief of violence is a revolution to help God in accomplishing his will.
The extremist is known to be someone who will take all of his beliefs to a very extreme level. They will sometimes tend to use force. They will occasionally use violence to show people that what they believe in is right even if they are doing things at an extreme level. Some people become extremists to support the government or their religion.
Fundamentalists are known to be literal in explaining and understanding the text that they have read. They will feel that the Bible will hold the absolute truth in all of the things that they should do in their lives. These are people that can be hard to be with, especially since they do not adapt to modern times.