Your utility owns and maintains the power lines that deliver your electricity. They charge you a fee to deliver power across their lines and also manage the billing for all of your electric services.
The electricity itself is generated and supplied by third-parties. Your Public Utilities Commission chooses a default supplier called the Basic Service or Standard Offer, but thanks to deregulation you can choose your own supplier.
Just like now you’ll receive one bill from your utility. The supply line on your bill will change from Basic Service/Standard Offer to the Provider Power company that services your area, and that’s it. There is no interruption of service, ever!