Pretend for example we have three identical aircraft with the exception of the mounting location of the wing. High mid, and low. What are the advantages/disadvantages of each type? 
In a grossly simplistic aerodynamic explanation you'll see differences in stability and drag. Generally speaking, a high wing is more stable than mid-wing which is more stable than low wing. This is why you'll see differences in dihedral angle (wings angled up on low wing planes) and anhedral angle (wings angled down on high wing planes). Mid wing planes typically have neither anhedral or dihedral. The designer can either increase stability (dihedral) or decrease stability (anhedral) to achieve the balance he wants. Regarding drag, the high and low wing tend to have a little bit less drag than the mid wing which has two roughly 90deg intersections between the wing and fuselage while the high and low wing designs have only one. These intersections create drag.
From a practical engineering standpoint, there are other reasons to select high, mid or low wings. Mid wing planes mean the spar passes directly through the fuselage creating issues with interior space (for the cockpit for instance) while making construction and repair more difficult. High and low wings can often be built as a single piece and overall construction is usually simpler. Also, you have to consider external stores and the size and location of the prop(s). A low wing plane needs shorter landing gear and can mount them in the wing while a mid or high wing plane means either very long landing gear in the wing or shorter gear mounted in the fuselage which makes for potentially narrow and less stable landing configuration. On the other hand, a bomber with engines in the wings may benefit from a mid or high mounted wing to provide the necessary prop clearance while having the benefit of good natural stability.
You can see strange results as designers attempt to mix and match these requirements. The F4U started out with a low wing with a bit of dihedral but this required very long and weak landing gear due to the large prop so the designer cranked the wing with lots of anhedral on the inboard portions of the wing and lots of dihedral on the outer portions. This mean shorter, stronger landing gear and the anhedral and dihedral "balanced" each other out. The A4 Skyhawk is an unusual arraignment also. It's a small, low wing jet that didn't have to worry about prop clearance yet it had extremely long and spindly landing gear. This was done so the aircraft could carry a large external load including a nuclear bomb.
