Stress Fractures
Stress Fractures are pretty self explanatory. They are very minor cracks in the bones of your foot, similar to shin splints, caused by repetitive small force traumas, or extended strenuous activity that your feet weren't quite prepared for. The reason people are susceptible to these fractures is by way of sex, ethnicity, and age. Many people have various different degrees of Bone Mineral Density.
Causes
In almost all cases, these are common when a person starts preparing for something, or begins playing a new sport. 9 times out of 10, this is associated with doing something you don't do often, or start preparing for something that will be very physically demanding. The prime example of this is one from my father, Tom. He started training for a triathlon after years of sitting in a cubicle. Almost immediately he had severe shin splints and stress fractures in his foot.
This injury is commonly associated with running, jogging, jumping, marching, etc.
Treatment
The only real effective method of cure is decreased use of what limb is fractured and resting it more than usual. Current soreness can be treated with Ibuprofen and a bag of ice wrapped in a thin cloth.
Note: For full recovery of these fractures, don't put any strain on the fractured bone and rest for a while longer than you think is needed.