Foundations are generally only as good as the soil that they are in contact with or perched above.  Soil that contains a high content of clay can expand and contract and move structures with relative ease.  This type is know as "expansive" or "swelling" soil, and is considered to be the second leading cause of property damage in the United States.  As a homeowner, you might be looking at expansive soil if you happen to see cracks or fissures in the soil around your foundation, expanisve soil is also capable of creating cracks in house walls and other hard surfaces. Apart from the damage caused to structures by expanisve soils, they all remain vulnerable to stress, stress from wind, rain and natural disasters, and even from the wear and tear of time. 

There are two basic types of foundations: slab-on-grade and raised, whether raised over a crawlspace or a basement.

Slab-on-grade foundations are quite common and are typically found in climates where heavy snowfall and prolonged rain do not create problems.  Many slabs are made with a single pour of concrete, called monolithic slabs, in which bolts are placed when the concrete is still wet.  Cracks in slabs are common and are typically discovered after people move into a house and decide to replace the carpeting and the padding.  The cracks typically result from shrinkage or the natural curing process, and are usually not structurally threatening.  Of course, there are also cracks that result from soil movement or seismic activity, and it is wise to seal them with a non-shrink grout, to lessen the possibility of moisture intrusion, and particularly those in older slabs that are not likely to have a moisture barrier beneath them.  Maintaining a slab foundation is relatively easy: keep water away from it; make sure that surfaces and any surrounding soil slopes away for at least six feet, and if the house does not have gutters, install them.

Raised foundations vary considerably in the amount that they are raised. They are found in all climate zones, but particularly in zones where moisture from rain and snow and thermal extremes could have an adverse affect on the living space.  Raised foundations have crawlspaces that need to be cleaned and secure from pests and rodents and should be inspected periodically for evidence of water and waste pipe leaks, dry rot, and termite activity.  Screended-access is generally available on the outside to facilitate the service of the ducts, water and waste pipes, electrical conduits, and telephone and TV cables.  However, such screens also allow for an exchange of air, aided by a series of five by fourteen inch ventilation ports at intervals along the plate line.  For aesthetic reasons, people are tempted to put fancy covers over the vents ports, but anything that reduces ventilation is ill-advised.

All cracks result from movement, and sometimes serious movement. Most cracks are not structurally significant, but all confirm movement of one type or another.  Nevertheless, all essentially non-structural cracks should be sealed with caulk or non-shrink grout depending on the surface and as determined by the specialist.  Keep in mind that cracks with offsets in an pedestrian surface can also be trip-hazards and should be repaired before an accident occurs. 

It's worth repeating that all cracks are result of movement and all need to be noted and taken seriously. When in doubt, have a specialist check it out.