The First Law of Thermodynamics tells us how energy is conserved in a thermodynamic system.  If we consider an isolated system, where the system is separated from its surrounding environment by a boundary that doesn’t allow energy to pass through it, the first law states that the total energy of the system doesn’t change.  The Second Law of Thermodynamics, on the other hand, introduces the new concept of entropy and describes how the energy within a system will arrange itself spontaneously.  If our system is isolated and allowed to evolve spontaneously, it will evolve so that the entropy of the system increases.  Stated simply, the entropy of an isolated system will either increase or remain the same depending on whether a process is reversible or irreversible.  For all real world processes, the entropy of a system increases.