I am a general contractor. ALL concrete cracks, it is a matter of how large the cracks(side to side movement) are and whether there is deflection(up/down movement) or not. generally cracks smaller than 1/8″ are normal. Larger cracks or areas showing deflection can indicate potentially very costly repairs.

Most of the time the cause movement due to poor soil preparation(not enough compaction or not accounting for expansive soil) prior to slab placement, but it could also be an insufficient slab design for the structure(worst case), or heaving due to tree root intrusion.

I have done slab repairs – they aren’t for the do it yourselfer. If you are looking at a property with cracks in the foundation you should hire an engineer to get a look at the slab/soil.