Why do we grind coffee?

By grinding the coffee bean we are increasing the surface area of the coffee which enhances the ability of heated water to extract oils and enhance dissolved soluble flavors. So with that being said the process by which we extract the coffee will dictate the grind of the coffee bean. The more course the grind the longer the water must be in contact with the coffee to properly extract that beautiful coffee flavor. The finer the grind the shorter duration the water must be in contact with the grounds. If your coffee is overly bitter, most likely you over extracted your coffee, while if your coffee is sour and flat you most likely under extracted it. Bottom line though is you like your coffee how you like it so the above is not hard fast rules but rather a base of knowledge.
Below I will discuss 5 basic grind levels and methods of extraction for each.

Course Grind

Preferred extraction method: French Press, Cold brews
Long duration contact with water
Agitation (Stirring)

Medium Grind

Preferred extraction method: Flat bottom filter drip pots
Medium duration contact with water

Medium Fine Grind

Preferred extraction method: Conical filter drip pots, Pour over
Medium duration contact with water

Fine Grind

Preferred extraction method: Espresso
Short duration forced extraction

Extra Fine Grind

And here is where we break the rules. Turkish/Greek coffee uses very fine ground coffee. The coffee grounds are poured into the heated water, stirred and reheated to cause the grounds to settle on the bottom. The brew is then poured into a cup very carefully as to not disturb the grounds.