A "grid-tied" system is actually a "Utility-connected system" also called "grid-connected", or "grid-tied" systems, are for homes or commercial buildings that are connected to an electric utility. They are designed to provide a modest part to all of the building's total electricity needs. Advances in solar power electronics make it relatively easy to connect a solar electric system to the utility.
Energy generated by such a system is first used within the home, and surplus power is "pushed" onto the utility's wires. In many states of the U.S., local utilities have "net-metering," which allows a homeowner's meter to spin backwards when his or her electricity is pushed back onto the grid. When this happens, the utility buys electricity from the homeowner, instead of the other way around! (And the utility pays the homeowner for the retail value of the electricity. Without net-metering, the utility would be forced by law to buy electricity from the homeowner, (an independent producer), at wholesale.)
A draw-back of connecting your PV system to the grid (and using the grid as "storage") is that when your area suffers a power outage, your PV system automatically shuts off. (This is done intentionally, in order to protect people working on the lines from live electricity.) To avoid this problem, many people introduce batteries to their grid-tied system, which provide power in the event of a utility power outage.