The most common cause for a crack to appear in your foundation is the natural settlement of the foundation over time. Other causes include, but are not limited to:
Malfunctioning weeping tile
Freeze/thaw seasons
Poor drainage in the yard
Pour line in the concrete
Unusual ground pressure pressing against the foundation wall
Most cracks that appear are vertical and are easier to repair than horizontal cracks. Horizontal cracks are serious, and repairs should be completed ASAP as they indicate a problematic structural issue with the house.