In a recent "Beyond Bitcoin" podcast somebody made it really easy for me to understand the token relationships and roles in the Steem economic system.

There are 3 basic characteristics that a "Money" should have are:
1- A medium of exchange
2- A unit of account
3- A store of value

In the Steem economic system these characteristics of money are separated into 3 different and related tokens.
1- A medium of exchange = Steem
2- A unit of account = Steem Dollar
3- A store of value = Steem Power

Looking at it from this perspective, you can easily see the true genius of the Steem system.