One type of basement floor cracks you may find is in floating floors. What is a floating floor? Well, the foundation and walls are poured first; then gravel is brought in and compacted. The soil under the gravel is compacted as well. Then plastic is put down. Once this is all done, the concrete floor is poured to the basement walls.
The good news is that most residential construction is done this way, which means most cracks are not structural. Most basement floor cracks are caused by poor technique. The ground is not compacted, the gravel is not compacted, and possibly plastic down was not put down.
These cracks can be fixed by putting a material within the crack; it is a two-part material that welds it together. Most residential construction concrete floors are separate structures than the walls so that they can move and adjust during temperature changes.