Every thermodynamic system exists in a particular state. A thermodynamic cycle occurs when a system is taken through a series of different states, and finally returned to its initial state. In the process of going through this cycle, the system may perform work on its surroundings, thereby acting as a heat engine.
A heat engine acts by transferring energy from a warm region to a cool region and, in the process, converting energy to mechanical work. A heat engine is a reversible process. However, it would take an outside force to reverse any process and will not be an efficient process at all. The system may be worked upon by an external force, and in the process, it can transfer thermal energy from a cooler system to a warmer one. One such reversible process is a refrigerator; however it takes a lot of power and energy to keep things cooled down. Efficiency becomes very important in such an instance.
A special case of a thermodynamic cycle is the Carnot cycle. The Carnot Engine is the most efficient heat engine possible in nature.