Are horizontal cracks in foundation walls more serious that vertical cracks?

There are many factors that come into consideration when considering the "seriousness" of a crack in a foundation wall - crack length, direction, width, height/thickness of the wall, steel reinforcement within the wall, etc. In a nutshell, horizontal cracks tend to be of more concern than vertical cracks. The best thing to do would be to contact a structural engineer, who can evaluate your situation.