It's cause is an imbalance in the cycle of bone re-building. Like many tissues in the body, bone is continually removed and re-built. Osteoporosis is when the balance of removal and renewal shift towards removal, and the bones lose density. It is pretty much a normal part of aging in people eating the 'standard western diet', and becomes a problem when there is a marked degree of loss of bone density.

More women in the West are affected by osteoporosis than men. In a typical Western country such as New Zealand 56% of women in the over 60 age-group will be affected by osteoporosis, but only 29% of men in that age group will be affected. It is usually considered that those women who will be affected by osteoporosis develop the shift in calcium balance just before or during menopause.
 
The main risk is fracture of bones, which is co-related to the amount of overall loss of bone density. Loss of density can be slight through to half the pre-existing density, depending on the severity of the imbalance over time. A 10% loss can double the risk of spinal fractures in a fall, and there is 2.5 times the risk of a fractured femur. Hip and spine are the two areas most commonly affected by osteoporosis.
 
Apart from a increasing 'stoop', generally most apparent at the top of the spine (a result of crushing of the vertebrae), the only other suspicious signs are fractures caused by seemingly minor traumas, recurring fractures, or perhaps unexplained back pain. A bone density scan is able to establish whether or not osteoporosis is present. While younger people are much less likely to have osteoporosis, it is still seen in the 'not-that-old'
 
First, the det must contain sufficient calcium. But calcium absorbtion is, in most western diets, more important. People vary in their ablity to absorb the calcium that is in their diet. One study of woman at and around menopause concluded that the proportion of dietary fibre and fat in the diet were the strongest factors predicting calcium absorbtion (women with the lowest amount of fat in their diets had 19% lower fractional calcium absorption than women with the highest fat diets).[ref] While it was once thought that the high protein and phosphorus content of the typical Western diet inhibited full calcium absorption, it has been shown by a recent study that this is not so. In fact, the same study concluded that regardless of whether the estrogen status is high or low, neither high or low phosphorous or high or low protein intake had any influence on the absorbtion of calcium.[ref]  While this study (on a group of nuns) did not distinguish between protein source, an earlier study did. It showed that those with the highest animal protein intake (more than 91 grams of protein a day, mostly animal protein from fish, meat, eggs, or cheese) had the least risk of hip fracture. In fact, the women who ate less than 49 grams of animal protein a day (the 'recommended dietary allowance' for protein is 50 grams ) had five times the hip fractures as those eating more than 70 grams of animal protein a day.[ref] 
 
So a high fibre diet (especially from vegetables and nuts) coupled with adequate amounts of natural fats, and good helpings of animal protein in the form of meat, cheese, eggs and fish - a natural diet - seems protective; and ample calcium for the bodies needs will be taken in without needing 'supplements'.
 
References
Wolf, RL et al. 2000. 'Factors associated with calcium absorption efficiency in pre- and perimenopausal women.'
American  Journal of Clinical Nutrition. 2000. Vol 71. p 466-71.
 
Heaney, RP.2000. 'Dietary protein and phosphorous do not affect calcium absorption.'
American  Journal of Clinical Nutrition. 2000. Vol 72. p758-761.