There are many causes for cracking in concrete. Concrete goes through many changes as it hydrates or hardens. Allowing the concrete to harden and achieve strength before these stresses arise will eliminate a large portion of these cracks. Proper curing, a good mix design, use of shrinkage-reducing admixtures and the addition of fibers are all good ways to help minimize some types of cracking.
Concrete is very good in compression but poor in tension. Reinforcement allows the concrete structure to maintain its compressive integrity while still providing good resistance to tensile or flexural stresses.