Cracks form in concrete for many different reasons and can show themselves days or years after the pour. The most common cracks are known as plastic/shrinkage and movement. Plastic or shrinkage cracks form during the early hydration process and are created due to the concrete shrinking. Yes, it is normal for the concrete to shrink slightly. The analogy I use is if you leave a cup of pudding in the refrigerator uncovered, cracks will form in the top due to evaporation, similar to some of the characteristics of concrete. The second type, movement cracks, are rather self-explanatory; however, movement can be caused by a site not compacted properly leading to the concrete settling or by frost where the concrete is lifted at varying rates.