Hairline cracks should be monitored for changes. Cracks greater than a quarter of an inch in thickness should be evaluated by a professional.
Stair-step cracks indicate settling of the home.
Vertical cracks can allow water into the home.
Long horizontal cracks are worse than vertical cracks and should be looked at by a structural engineer.
Sometimes cracks in the foundation are created by normal settling, other times it indicates problems with the soil. Sometimes during construction, a portion or all of a home is built on fill-dirt that may give in later on down the road.
When evaluating a crack the newer the crack the more seriously you want to take it. If a crack has been around for decades and hasn't budged, then it is less likely to be a problem, but all cracks can allow water penetration which can lead to a larger crack. So, that's something you want to keep in mind. When in doubt call a professional, and have them evaluate and seal the crack and address the root cause of the crack.
Cracks accompanied by bowing of the wall should definitely be seen by a structural engineer.
In East Tennessee, we have clay dirt which expands and contracts with weather conditions increasing the likelihood of cracks in a foundation.
How to Repair a Crack in a Concrete Foundation - This Old House