Midsole: The foam piece between the upper and the outsole. This is usually made of CMEVA (compression molded EVA). The amazing thing about midsoles is that they differ in densities. Some are very soft while others are firm. This material can also be formed to just about any design a company wants. In recent years, midsoles have been designed to wrap around the heel of the shoe to add stability. Additionally, the midsole can be combined with plastic pieces to increase stability (see medial post below).
Medial Post: This is the area of the midsole on the inside heel portion that is made harder or more dense to prevent your heel from rolling towards the inside (pronation). Companies take different approaches to this theory. Some use a plastic piece and denser midsole materials or use denser materials alone. In either case, this is the feature you need if you pronate. The amount of medial post depends on the severity of your over-pronation. As the severity of overpronation increases, so do the medial posts.
Cushioning Technology: Air, Gel, adiprene, Hydroflow, Hexalite, DMX, Absorb are all terms the shoe companies use to advertise their cushioning technology. Designers place this technology in key areas to decrease the impact of running. Some shoes do not possess any technology, some have it only in the heel, some in the heel and forefoot and still others the full length of the shoe. What's right for you? Is one better than the other? What you want to do is purchase a shoe that has at the very least some form of technology in the heel. For those who feel the need for additional forefoot cushioning, purchase a model that has technology in the forefoot. A word of warning: the more technology a shoe has, the more it will cost.
Info from VS Athletics
Shoes Design Features (Midsole)
