What is the diff between analog ground and digital ground?

Typically digital circuits will pump a lot of fast transient currents into the ground. Since real grounds have resistance and inductance, this will produce a drop between the ground of the digital circuits and the power supply ground. This is the so called "ground bounce".

If an analog circuit ground shares ground path between the digital ground and the power supply ground, it will see this voltage drop. Depending on the common mode rejection of the analog circuit in question and the frequency components in the "ground bounce" this may or my not be an issue.

If it's an issue, the easy solution is to connect the analog ground directly to the power supply ground without sharing any of the path from digital ground to main power supply ground. This way the current spike induced drops in the digital ground don't show up in the analog ground. A good example would be ADC grounding for the digital and analog portions of the ADC chip.