The governance of an electric grid is a common states rights issue. Sometimes, a state decides its grid will be a monopoly, and that is called a regulated grid. A deregulated grid, therefore, is a grid where you can pick and choose who you buy your electricity from.
There are competing theories on which option is better. In theory, a regulated grid should be more efficient and offer greater consumer benefit. However, deregulated grids provide more flexible rate options than regulated grids, and so it is possible to find cost savings by changing your electricity provider. That can be a hassle, but depending on the size of your electric bill it can save a lot of money.
So, if you want to know the answer to whether you are on a deregulated grid or not, you might as well ask “Do I have the option to choose where I buy my electricity?” If the answer is yes, then you are on a deregulated grid.