The Fundamentalist Church of Jesus Christ of Latter-day Saints are not Mormons.  
 
They follow most of the teachings of the Church of Jesus Christ of Latter-day Saints but they currently and illegally practice polygamy (plural marriage) which violates the doctrine of the Church of Jesus Christ of Latter-day Saints.  
 
The founders of the FLDS are excommunicated Mormons who grouped together in formed there own religion.  
 
 
In the LDS Church we believe that only those who are called by God as his Prophet can receive revelation and lead the Church of Christ.  
 
The 4th President of the Church Wilford Woodruff received a revelation from God that the Church would be destroyed unless we obeyed all of God Commandments.  
 
At the time the Church still allowed plural marriages even though doing so was illegal (and we have several revelations that say obey the law of the land). The Prophet made and Official Declaration declaring that the Church would follow the law of the land and stop performing plural marriages.  
 
Some secretly continued performing plural marriages until 1909 when they were excommunicated upon discovery of the practice.  
 
These excommunicated members made there own Church and chose a false prophet to follow that would support there desires to continue to disobey the commandments of God.  
 
So the FLDS is not any more Mormon, then the Protestants are Catholic.  
 
The FLDS were excommunicated from the Church of Jesus Christ of Latter-day Saints because they continued to sin (by disobeying the commandment to obey the law) and refused to repent or stop there illegal practices.  
 
To call them Mormon is inaccurate since they are not part or ever will be part of The Church of Jesus Christ of Latter-day Saints unless they repent and cease there continued illegal acts.  
 
I thank you Mity for your question and hope that my answer helped you better understand the differences between the FLDS and the LDS Church and that the FLDS are not Mormons.