Grouping the Root Joint
 

So in this book I'm using it says to group the Root joint of the skeleton so it has another node on top of it. Then you rename the Root joint "Pelvis" and the group node "Body". Why is this done? What do you use the Pelvis for and what do you use the Body for?

this is done so you can tranlate all the components of the body together, like if you have ik on the legs or arms..you would also parent them under the body group, if you parent them to the root joint the ik stops working...