There are few structural foundation issues more telling than bowing foundation walls. Bowing walls can be a sign of serious structural damage, and can lead to total foundation wall collapse if it is not addressed.
Bowing walls are generally caused by excess soil pressure outside of the foundation walls. This pressure can be attributed to soil saturation, as well as soil freezing and expansion during the winter months. Bowing cinder block walls are especially common, as they have more structural weak points in the mortar joints.
A good rule of thumb is that a bow of 1″-3″ should require the attention of a foundation professional. Anything less than is within the acceptable limit, but should be monitored over time. A good way to measure the bowing on your foundation walls is by tying a weight to a string, and hanging it from the furthest bowed part of the wall (usually the center), to the floor. You may then measure the distance from the weight to the wall.