Sometimes a picture is better than a thousand words.
I am researching the Steem ecosystem and thought of this and had to share.
This picture is not the perfect analogy but still can tell some things to newcomers.
Users of Steemit, Chainbb and Esteem don't have to know who is pulling the train. They just get onto a wagon and go wherever they like. Getting onto the train is easy. Pay for the ticket. Open door. You are in. Wait for train o arrive at your destination.
"Steem engine" is the "machinery" that carries them all. It is a block chain, which can carry tremendous amount of data compared to others like bitcoin, ethereum. In a sense it is the machine with highest horse power.
STEEM POWER is kind of the "voting power" that has interest in this business of transportation. The holders of it are determining future of business.
STEEM coin is like the coal that is necessary for operation of the train. SBD is like wage for machinists.
Some tech talk: One can add another wagon to the train, tools are available for that. The wagons are "subchains" as 
This locomotive is available for everyone thanks to MIT license. You just have to plug your car and let it carry you.
You can join the existing running Steem public block chain train, or you can start another locomotive and another transportation business!