A grid-tied solar system is connected to the utility electrical grid.
There are two major benefits of this:
Still being connected to the grid means that you are supplied with electricity when the sun is not shining You can sell surplus electricity back to the grid and earn money
An off-grid system is the exact opposite – You are disconnected from the electrical grid. This is in the majority of cases not a financially viable solution, but in some areas where the distance to the grid is too far, an off-grid solution could be feasible. Batteries are need to store surplus electricity for times when it is really needed.
Another way of looking at a grid-tied solar system is that the utility grid is a massive battery – much more efficient than stand-alone energy storage systems.