I know its a big image, Its for a reason.

The left side of this hub is what we have all been refering to as a fixed side, or fixed hub. The outside part is smaller and reverse threaded then the inner part so that a lockring can be threaded on. A freewheel can be threaded on that side too.

The right right side is the free side. Since it doesnt have the stepped down part it can only take a freewheel, making it less useful.

This is a formula hub, they will be all you ever need. They make a variety of holes, and have a fixed/fixed, fixed/free and single sided hubs. Definatly the right choice.