Earth is what you stand on outdoors. This ground is necessary for electrical safety from the mains. How you connect to it is described in detail by the safety laws in each countries.
Analog ground is the reference point for analog circuits. This also is the power return point.
Digital ground is the reference point for digital circuits. This also is the power return point.
Where problems occur is when the ground is not a true conductor. Return circuit currents flowing through it cause voltage differences between different physical points. When circuits use these different points for their circuit ground, their outputs relative to other ground points are not the same.