NOTE: I will leave this post up for completeness's sake, but there is some outdated material in here (It's from January 2011!). Fair warning. ;)
When you walk into a running store, you're surrounded by walls of shoes. However, not every shoe on that wall is going to work for you and keep you healthy. What shoe you req
uire is dictated by your biomechanics, with the primary determinant being how much you pronate, or how much your foot rolls inward when it is in contact with the ground. Pronation isn't inherently evil, as it's your foot's natural cushioning mechanism. However, overpronation, or excessive inward roll, can cause injury problems if left uncorrected. Many runners display some degree of overpronation, though the degree varies by runner. On the other side of the spectrum is supination, also called under-pronation, which is when the foot doesn't roll enough and is unable to adequately cushion itself, which can stress the foot. Actual supination is fairly uncommon, and only around 10% of runners supinate.
Running shoes fall into three main categories: neutral, stability, and motion control. Neutral shoes are named as such because they have little to no stability features, though they do have good midsole cushioning. Neutral shoes are sometimes just called "cushioned shoes," though this name is misleading, since all three types of shoes provide cushioning. They are best suited for neutral pronators, who pronate ideally and need no stability features built into the shoe to control their foot motion, and for supinators, whose feet do not pronate enough to properly cushion their footstrike and require cushioning without stability features. Stability shoes usually have some type of medial post to provide stability, as well as good midsole cushioning. They are best suited for low to moderate overpronators, who could use some support to keep their foot from rolling inward too soon. The majority of runners do best in a stability shoe. Motion control shoes offer the greatest stability features, with higher density midsoles, extra medial support, and maximum heel counter control, and are often built on straight lasts with wide outsoles. They are best suited for severe overpronators or heavier moderate pronators. Other types of shoes (lightweight trainers, racing flats, etc) exist, but for the purposes of this post, only training shoes will be discussed (and flats and lightweight trainers fall into other categories anyway, e.g. Brooks T6 is a neutral flat while the Brooks Racer ST is a stability flat, and the Asics Speedstar is a neutral performance trainer while the Asics DS Trainer is a stability performance trainer).