Shoe designers are pretty savvy in that they know how to desing shoes to fit the typical foot and meet the demands of the sport.  However, there are a few parts of the shoe you should be familiar with and be able to evaluate prior to purchasing.

While this picture does not identify all the shoe parts listed below, it should help you identify them as you are trying to figure out just exactly what part of the shoe is being discussed.

Outer-Sole
    The outsole is the treaded layer glued to the bottom of the midsole. It resists wear, provides traction, and absorbs shock. The outsole is usually blown rubber, hard carbon rubber, or a combination.  Blown rubber is the lightest, but not as durable as carbon.  Carbon rubber is more durable, but is heavier and stiffer than blown rubber. There isn't a reliable way to differentiate between carbon and blown rubber, so determine which you prefer and ask to see the specification sheets at the store. Many shoes have a combination of carbon rubber in the high-wear areas of the rearfoot and cushier blown rubber in the forefoot for a softer ride.  Stud or waffle outsoles are good for running on dirt or grass because they improve traction and stability. Ripple soles are better for running on asphalt or cement surfaces.

    Look for one of two kinds of outsole rubber--carbon rubber or blown rubber--or a combination of the two.
 

Mid-Sole
    The midsole is located between the outsole and the upper. It is the most important part of any running shoe. It controls excessive foot motion and provides cushioning and shock absorption.  The primary materials used in midsoles are EVA (ethylene vinyl acetate) and PU (polyurethane). EVA is a foam that is light and has good cushioning, but breaks down fairly quickly. Compression-molded EVA is harder but more durable. PU, also a foam, is denser, heavier, and more durable than EVA.  Generally, bigger runners do well with polyurethane midsoles. EVA is more common because of its lightness and more cushioned feel.

    Most shoes are also cushioned with gel, foam or various manufacturer-specific technologies that are encapsulated in the midsole. This cushioning lasts longer than previous methods and often adds stability as well as shock absorption.
 

Stabilizing Material
    This is usually located within the midsole on the inside of the shoe.  When it is present, it looks like a different type of material.  Look for stabilizing devices that reduce overpronation. Stabilizing technologies are almost always in the shoe's midsole on the medial side (the arch side). Many shoes have firmer densities of midsole foam to combat overpronation. Some also have an external device such as a footbridge or a plastic medial post molded within a second midsole density.
 

Last
    This is the form around which the shoe is built, and is usually classified as either straight, semicurved, or curved. A straight lasted shoe is filled in on the inside/medial side of the shoe to increase stability for people with flatter feet. A semicurved last is designed for the average or normal foot. There is a small curve on the medial side of the foot to fit a normal arch.  A curved last is built for people with an abnormally high arch or for runners who do not pronate as much. The curved last is built with a larger curve on the medial side of the shoe and has a wider outside portion of the shoe to provide more forefoot stability. Curved lasts are made for faster runners who are on the fronts of their feet or toes.
 

Heel Counter
    The heel counter is a rigid or semirigid, moldable, external strengthening device in the heel area of the upper. Its purpose is to control and stabilize the wearer's heel inside the shoe and minimize excessive supination or pronation of the foot.  It must be rigid and durable to support and stabilize the heel. Because the internal heel counter material is thin and tends to lose its stiffness, an external counter is usually placed between the midsole and base of the heel counter.  A wedge, which adds height to the heel, enhances the shoe's ability to absorb shock and reduces strains. While this can ease problems like Achilles tendinitis, a shoe with a higher heel may feel less stable.

    Look for a snug yet comfortable fit. Too loose a fit can cause blisters on your heels. If you require extra stability, look for a stiff heel counter or an external heel counter or ring that wraps around the outside of the heel.

Upper
    The material sewn into the midsole or outsole comprises the upper. The largest pieces of the upper are called the quarters (sides of the upper) and the vamp (top surface of the shoe upper).  The most popular upper materials include leather, woven soft nylon (taffeta), and coarser-grade nylon knitted into a mesh configuration for strength and breath-ability.

    Look for an upper (the material that encloses the foot) that fits properly and allows good ventilation. It helps the shoe stabilize the foot.
 

Toe Box
    This is the front part of the upper of a shoe, where the balls of the feet and toes are. The width and height of the toe box will vary depending on the brand and model. An improperly fitted toe box can lead to several toe irritations like bunions, hammertoes, and blisters.

Tongue
    Look for a tongue that's thick enough to protect the top of the foot from the pressure of the laces, yet not so long that it irritates your leg just above the ankle.