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.