Actually if you look up Fundamentalist (Fundamentalism), the definition is ... 1. (sometimes initial capital letter) a movement in American Protestantism that arose in the early part of the 20th century in reaction to modernism and that stresses the infallibility of the Bible not only in matters of faith and morals but also as a literal historical record, holding as essential to Christian faith belief in such doctrines as the creation of the world, the virgin birth, physical resurrection, atonement by the sacrificial death of Christ, and the Second Coming.  
reference: http://dictionary.reference.com/browse/fundamentalist  
 
So the way I see it ... they're basic philosophy is no different than ANY Christian based religion ... their practice is different. But then again, all religions practice differently.

Thank you for asking the question! A lot of the people who don't know the answer to the question, just assume that FLDS and Mormons are the same people. Although I am not LDS and in fact I am not a Christian, I am very often troubled by the stereotypes that people have about LDS, and I am especially troubled when people make assumptions without checking the facts. Your questions indicate that you are the type of person who checks out the facts first before forming a conclusion.  
I can't answer from an LDS perspective to give you details about how the Mormons are different from FLDS. So, I can only answer from my own perspective.  
 
1. The Catholic Church at one time was the main Christian church, and most Protestant churches are offshoots from the Catholic Church. This includes Baptist, Lutheran, Methodist, COGIC, Foursquare, Pentecostal, etc. Of course, if you asked "aren't Catholics and Pentecostals the same people?" then Pentecostals AND Catholics would probably feel offended, because they are so different and they don't even want anyone to THINK that they are the same.  
I think its the same regarding FLDS and Mormon. In fact, as the Mormon church grows, I expect there will be many offshoots just like there are many major religions now that are offshoots of the Catholic religion.  
The recent news about the FLDS situation has caused me to reflect on the years when I didn't know much about Mormons but I THOUGHT I knew plenty. Now that I've spent years learning everything I can about Mormons, I realize that I was once very ignorant but it hadn't stopped me from making all kinds of ill-informed statements and judgments. I came to this askmamormon.com website, in fact, because I feel very depressed and guilty about my former ignorance and I am seeking ways to "repent".  
So, again, thanks for the question!

Thanks activista for trying to get more knowledge! That's great!  
 
As I have been watching the news, I think it is important to point out that the Mainstream Mormon church does not practice polygamy, the FLDS do. (For more info on the subject, read the polygamy related threads on this site).  
 
The FLDS church is an offshoot of the Mainstream Mormon Church (although i don't know how long ago that took place), and many of their practices and beliefs are so far apart from the Mainstream Mormon Church, that I don't think that they would even consider themselves as "Mormon" Or, maybe they do, and consider themselves the "Pure Mormons". Dunno.  
 
But they definately are different.

FLDS is a splinter group of the mainstream LDS church. The FLDS splintered off when the church disavowed polygamy in the 1890's. 
 
FLDS and LDS are nothing alike in culture, and only somewhat similar in doctrine. LDS live among the general population of the world, while FLDS live in compounds or small, insular communities. 
 
Theologically, LDS have chosen to focus on the mainstream commonalities between Mormonism and Christianity, while FLDS have tended to focus on the differences and oddities between the two. 
 
I lived near FLDS for much of my life. They believe in their faith, but are very distrusting of others and often distort the truth to protect their way of life and beliefs.

..That we don't have an exact explanation of what their doctrines are. From what I have observed thus far, it seems that they are very "to themselves" as well, as W.E.B. referred to. 
 
The best resource I have found regarding their beliefs is a website at www.fldstruth.com. I have read through it, but couldn't really find any difinitive beliefs spelled out, or reasons for their practices that we view as "different".

If the Book of Mormon contains the fullness of the everlasting gospel, why does th LDS Church need additional works?

The fulness of the Gospel is the things we need on this earth (authority through the proper priesthood, ordinances, scriptures, etc.) for our salvation. Fulness does not mean the complete, entire knowledge and wisdom that God has. Although we have the fulness of the Gospel for our salvation - we need additional light and knowledge to help us and guide us.  
 
The same analogy could be given with the New Testament. Surely Jesus taught the fulness of the gospel. So why do we have the teachings of the Apostles after Jesus was crucified? It is because we always need light and truth to help us. We always want help from Our Heavenly Father in whatever situation or circumstances we may find ourselves.