Cracks in concrete are not always a problem. When concrete dries it often cracks. These cracks are called shrinkage cracks. These are typical and can be expected and usually don't cause any problems as long as water is not allowed into the crack, especially freezing water. These cracks are sometimes called "hairline" cracks because they are so small/fine.
The kind of cracks that require attention are horizontal and vertical cracks. These types of cracks are found on a vertical surfaces (foundation walls) and should get our attention.
Horizontal cracks are often created from lateral pressure exerted against the wall from hydrostatic force or backfill that was put in too soon after the wall was poured/formed. Hydrostatic pressure is created when the soil on the exterior of the home is saturated and applies pressure to the wall. This force is exacerbated when the ground freezes. That is why it is so important to keep downspouts clear and downspout extensions directed away from the home at least 5-6 feet. And to maintain a positive grade (sloping ground away from the home) around the foundation.
Vertical cracks can be an indication of footing issues. When the footings of a home rise or sink it puts pressure on the wall in a vertical load path and can cause vertical or diagonal cracks to appear. These are cracks that should also get our attention and be investigated. It is important to know what is happening and correct the issue before further damage is done. It may be the soil is sinking under the footing or is expanding. Either way, have it looked at by a foundation expert for further clarification and course of action/repair.
This is a picture of horizontal cracking in a crawlspace foundation wall. This is more than likely the cause of hydrostatic pressure exerted on the wall due to missing downspout extensions on the downspouts just outside the wall in the front yard. This may not have happened if the extensions were in place directing water away from the foundation as they should.