Once electricity is generated by the generator, it is then supplied to different regions / geographical locations and this is the responsibility of a distributor. A distributor is the one responsible for maintaining the electricity poles, wires, gas pipelines etc. In other words they take care of the entire physical infrastructure involved in sending electricity / gas to your home / business.
On the other hand, a retailer is the one that buys electricity / gas from the generators and further sells it to you at a charge mutually decided by the retailer and energy regulator. So the electricity / gas bill that you get is from the retailer you are with. In a deregulated market as in VIC, SA, NSW and QLD it is possible for customers to switch their retailer, but by doing that they still remain with the same distributor as far as they do not change their address. So irrespective of any distribution network you may be, you have the option to choose a retailer of your choice.