Most PV solar systems are on-grid systems. See, for details: On-grid Solar and wind systems.
A stand alone PV energy system is not connected to a wider public or private grid. In this case it responds only to the needs of its owner. The stand alone systems is typicallu more expensive due to the required set of batteries for storing the energy.
Stand-alone PV solutions are popular in remote areas, where the connection to electric grids is expensive or impossible.