To begin, one ought to understand the traditional meaning of a "fundamentalist" with regards to religion. Historically, a "fundamentalist" was one who adhered to the core doctrines and/or teachings within the system of thought in question. Today's post-modern era has redefined the label as a strict following to a set of principals, which are deemed intolerant (exclusivism) towards other religous institutions and movements. Hence, when one is referred to as a "fundamentalist" using the second definition, it is often rejected with ferver because of the negative connotation. Being exclusive with truth claims, in the eyes of post-modernity, seems vicious and wildly politically incorrect. But I'm concerned strictly with the first definition (Historical and traditional) when it comes to Islamic terrorists. Does the modern Muslim follow in the footsteps of Mohammad's teachings? If so, what is the relevant difference between a fundamentalist Christian and fundamentalist Muslims?
Historically, traditional Muslims have stringently opposed all practices not sanctioned by the Qur'an. Currently, there are Muslim scholars who claim that Islam in a religion of peace and often cite verses of the Qur'an to back their assertions. Prolific Christian teachers, such as Billy Graham, teach that Islam is a religion that commands undifferentiated slaughter. How do we make sense of this contradictory dichotomy? What does the Qur'an teach when it comes to people of other faiths? There are two methods to understand where the religion of Islam stands.
First, we must use sound interpretive insight (hermeneutics) to look within the pages of the Qur'an to search for explicit or implicit statements confirming a core teaching-- a fundamental teaching. Muhammad says in the 52nd book of the Sahih Bukhari,
"...If somebody (a Muslim) discards his religion, kill him." 
With this alone, one can establish reliable convictions that the author of this text advocates the unjustified taking of life because of either a conversion to another faith or the unfastening of the Muslim faith altogether. Religious freedom truly is a virtue we ought to be more thankful of--at least for the moment. In other parts of the Qur'an, Sura 8:39 and 9:5 say,
8:39 "Make war on them until idolatry shall cease and God's religion [i.e. Islam] shall reign supreme."
9:5 "So when the sacred months have passed away, then slay the idolaters wherever you find them, and take them captives and besiege them and lie in wait for them in every ambush, then if they repent and keep up prayer and pay the poor-rate, leave their way free to them."(emphasis added) 
To summarize the arguments and evidence presented earlier, the Qur'an teaches not only vehement principles as orthodoxy, but that those tenets ought to be implemented in deed. Addendum, Islam, based on its fundamental teachings, is a confirmed "war religion". Ergo, Muslim fundamentalists hold to life-taking doctrines on the basis of worldview differences. Thankfully, the majority of Muslims take a Moderate stance and do not subscribe to the fundamental tenets of Islamic ideology: global Islam by force. They have liberalized the Qur'an by adopting a different hermeneutic that denies the authoratative Qur'an and Haddith sources and, in its place, promote peace as a bottomline virtue. Historically-savvy hermeneutics ought to be encouraged to Muslims (and Christians, for that matter) for an accurate understanding when reading the texts.
To conclude, remember what the question I proposed apropos the definitions of a fundamentalist: "Does the modern Muslim follow in the footsteps of Mohammad's teachings? If so, what is the relevant difference between a fundamentalist Christian and fundamentalist Muslims?" The devout Muslim, ie. extremist terrorist, are prime examples of what follows from taking Mohammad's teachings at face value. But I'm highly speculative that the majority of Muslims embrace classical Islamic theology. To be honest, the average modern Muslim suffers from illiteracy when it comes to the Qur'an and the historical context it was written in-- much equivalent to the vass majority of Christians with the bible and its cultural setting. Fundamentalism is fatal for the Muslim, but vital for the Christian. Why? Because, as we've seen (and are currently viewing in current events), orthodox Islamic theology produces extremist radicals fused to the ideology of its founder. The fundamentals of Christianity establish a religion of ultimate redemption, reconciliation, and glory. Christians holding to the tenets of Christianity ought to be hopeful for the products these fundamentals yield. Au contraire, Muslims faithfully living out Mohammad's words carry the burden of reconciling their "war religion" in contrast to values of religious tolerance and freedom. Ultimately, their is an inherent contradiction that will plague this dilemma for originalist Muslims.
1.) Sahih Bukhari: Volume 4, Book 52, Number 260
2.) Trifkovic, Serge. The Sword of the Prophet
- Donner, Fred M. "The Sources of Islamic Conceptions of War". Chapter in Just War and Jihad: Historical and Theoretical Perspectives on War and Peace in Western and Islamic Traditions. John Kelsay and James Turner Johnson Eds. Greenwood Press: New York, 1991.
- Campbell, Charlie. "Islam"; Calvary Chapel Lectures
Cross-Blogged at Apologia Christi