Difference between Ground and Digital Ground

Digital Ground usually refers to the reference voltage of digital logic ICs. This means that no analog signals are coupled into this reference plane.

You can often see analog and digital ground where digital ground will be the reference for any logic and analog ground will be the return path for any analog circuits.

Imagine an Atmel AT32UC3C. It has both an analog and a digital ground. The analog ground is intended as reference voltage for the ADCs and DACs or Analog Compare interfaces whereas all other logic is considered to be digital.

You will also often see that analog and digital ground are connected to each other directly or via a ferrite. "Ground" is just a name for the reference net in your circuit.

The main reason for having multiple (different) grounds is that effects of one application do not affect the other (e.g. analog vs. digital).

It's not uncommon to see multiple grounds in a circuit and it's normal for these to be joined together at some point: either directly, or with a zero ohm resistor, or small ferrite.

The reason you have multiple grounds is to control the return path for various currents. Digital circuits for example may contain fast edges and so have the potential of introducing significant noise. Separating the digital from analog ground allows you to keep the analog signals clean of any noise from the digital switching. The digital ground will contain noise but this matters less as you are only interested in logic high or low; the exact voltage is less important.

The two grounds will be "starred" together at some point perhaps directly at the capacitor lead of the power supply.