Grid-tied PV is a solar electricity system that is connected to your utility meter. Solar generated power from grid-tied systems is first used by your home or business needs while any excess capacity is sent to the utility grid (sold at retail price to the utility). During the evening or times when your power demand exceeds your solar production, your home will run off of utility power.
Stand alone or off-grid PV systems are not connected to the grid and rely on batteries for storage of electrical power. These systems are significantly more expensive and require more maintenance than grid-tied systems, but they are the ultimate form of energy independence. Stand alone systems are commonly used in remote areas where utility power is not available.