Common grace is the favor that God gives to all people. Generally, it is manifested in the way God takes care of all people by providing for them sunshine, rain, shelter, food, government, laws, general health, etc.
Common grace extends to every human alive.
On the other hand, saving grace is that favor from God expressed upon those whom He has chosen. In him we were also chosen, having been predestined according to the plan of him who works out everything in conformity with the purpose of his will. We, the chosen, have had the blessing of saving grace bestowed on us in abundance. Saving grace is based upon the love of God manifested in the sacrifice of Jesus for our sins. Grace is not commodity or a substance.