Grid
Simply put, your electrical system is still tied to the national power grid. A grid-tied system brings in electricity from the national power grid when your solar power system is not producing electricity, i.e. at night. There is no need for batteries unless brownouts in your area are a constant problem. A grid-tied system ensures that you have electricity whenever you need it.
Off Grid
An off-Grid solar power system is entirely separated from the national power grid. You will need batteries to store power generated from your solar panels. This type of system is commonly found in rural and distant areas that have little or no access to the national power grid.