A stress fracture is when a particular bone is damaged, but not broken through and through like a traditional fracture. Don't let this fool you though, stress fractures are real fractures! A great way I always explain this is as follows: A stress fracture is similar to the shell of a hard boiled egg if tapped on a hard countertop. There is some splintering to the shell, but it doesn't break in half and let all the goo out! If the egg were raw and hit on the side of a bowl or on a counter, the shell breaks open and the egg will leak out. While the shell remains intact in the hard boiled egg scenario, there is still damage present. The same is true in stress fractures. Just because there isn't an obvious break through the bone, there is damage which needs to be repaired.
If a stress fracture is not protected and allowed to rest and repair, the stress fracture can spread and worsen until the bone is weak enough that it completes the break and becomes a true fracture. The difficult thing with stress fractures is that they do not always show up on an x-ray. When the initial injury occurs, the damage usually does not present itself on the x-ray. It is only after the bone begins to remodel that you will be able to appreciate some changes in the appearance of the bone which signifies the reparative process.
Another note about stress fractures and why they are different from traditional fractures is the manner in which they occur. Simply stated, a typical fracture occurs when there is a high amount of energy placed through the bone in a short period of time and this results in a break. A stress fracture occurs when there is a moderate amount of energy placed through the bone over a long period of time repetitively. Stress fractures commonly occur when a person begins a new, demanding exercise and does not properly train. They also occur in people who have compromised bone density and increase their activity.
**Notice the faint "fuzzy" appearance in the picture on the left of a stress fracture, compared to the obvious break seen in the bone on the right indicated by the yellow arrow**