Muscle cramps are simple -- they are tight muscles that won't relax. To understand why this happens, you have to understand how muscles work: calcium causes muscles to contract, and magnesium causes them to relax. Due to diets that are too high in dairy foods, like milk, cheese and yogurt, excess calcium is a very common problem in the American diet. What makes it worse is calcium supplements, especially when they are combined with Vitamin D, as this greatly increases the absorption of calcium. Vitamin D is very important, but when you take it with a calcium supplement, it is quite easy to get a calcium overload that can cause cramps, or make existing cramps worse.
In addition, as we age, the body generally "calcifies". This means that the tissues, including muscles, tend to accumulate calcium and "harden". This is part of the reason why cramps become very common among people age 50 and older.
If you suffer from muscle cramps, you need to try reducing your calcium intake, and add a high absorbing magnesium supplement, which counteracts the effects of the excess calcium and helps muscles relax.