Well people would be here all day telling you that, I can give you some basic inormation though.
The UFC is known as the biggest MMA orginization but there are also very many other big ones the second biggest being Strikeforce or PRIDE. DREAM and Bellator are also getting large
The UFC's champions are:
HW: Brock Lesnar
LHW: Mauricio "Shogun" Rua (Its actually Loyoto Machida but we all know he lost)
MW: Anderson Silva
WW: Georges St.Pierre
LW: BJ Penn
All of those champions are extremely dominant and I can't see any of them losing for a long long time.
Oh and FEDOR is the greatest. Remember that
If you have any questions in specific ask and you shall recieve. If you feel you need to constantly ask questions PM me and I will always be willing to answer them