A Grid-Tie inverter supplements your mains utility power with solar power. For this to happen the electricity produced by your solar panels is converted from direct current (DC) to alternating current (AC) by the grid-tie inverter.
The inverter will then synchronise the power being produced by the panels and the power that you are using and lower or zero your consumption. If allowed by your mains supplier, over production can also be fed back into the grid, for which you will then receive credits (at a much lower rate though!).
Q. How will I know if a grid-tie system is what I need? A. When your main power usage is during the day (i.e. when the sun shines!).
Q. Can I add batteries on a grid-tie system? A. Yes, but your inverter might need to be replaced with a so-called hybrid or island inverter.
Q. What happens if the main utility power fails if I have a grid-tie system? A. Unfortunately, the grid-tie system needs a grid to function (it needs the AC 50 cps power cycle to synch to) so what will happen is that the whole system will shut down and as soon as the utility grid power comes back on it will restart itself. Note that an AC generator can be used to replace the grid power and the grid tie will then synch to the generator, saving you fuel.