1. System and Control Volume
In Thermodynamics the devices such as heat engines and refrigerators are systems with an input and an output.

2. Work
WORK: The IC Engine, in an automobile drives the vehicle by consuming a fuel, petrol in most cases. The input is fuel, output is the power from the wheels which can be connected to a pulley to raise weights continuously. This net effect is called Work, Positive when the system does work on the surroundings-- the wheels or the weight. If the weight went down, the system would not be doing work but receiving work, and it would be negative work.
Work, therefore is done when the overall effect of the system on the surroundings is equivalent to the raising of a weight W by a height h or Wh in Newton Metres.

3. Heat
In a Steam Power Plant, Electricity is generated, which again is in Watts or Nm/s, that can drive a motor and raise weights.
Any energy that crosses the boundary between system and surroundings that is not related to raising weights is Heat. In a steam plant it is the energy that comes from burning coal, the input heat, positive Q by convention, in N m. If the system liberated heat as a condenser in a steam plant would the heat involved would be negative heat.

4. Systems and Control Volume
The word system is very commonly used in thermodynamics; let us know what it is. Certain quantity of matter or the space which is under thermodynamic study or analysis is called as system. Let us say for example we are studying the engine of the vehicle, in this case engine is called as the system. Similarly, the other examples of system can be refrigerator, air-conditioner, washing machine, heat exchanger, a utensil with hot water etc.
Now, let us suppose that we have to analyze the performance of engine in different conditions. Here, we will feed the engine with fuels of different grades and load it with different loads to find out its efficiency. We will also find its performance during idling, acceleration, varying speed, slow speed and high speed. A thorough analysis of the engine is carried out; hence it is called as system.

5. OPEN, CLOSED, ISOLATED Systems

6. Closed System
The system in which the transfer of energy takes place across its boundary with the surrounding, but no transfer of mass takes place is called as closed system. The closed system is fixed mass system. The fluid like air or gas being compressed in the piston and cylinder arrangement is an example of the closed system.

7. Control Volume

8. Isolated System
No energy exchange or mass exchange with outside. Normally System and its Surroundings taken together form an isolated system. When we talk of the environmental changes, these are because of the cumulative effect of all systems on the universe.

9. Cycle
In Refrigerators and Thermal Power Plants, there is an associated working fluid. This is the medium of energy exchange. In a refrigerator, Freon in a refrigerator and water in a thermal power plant that undergoes repeated condensation and evaporation, compression and expansion. Freon uses electricity and Water uses coal to produce cooling and electricity respectively. While the coal is exhausted the same water keeps circulating in steam plants, just like our blood does its job of helping us getting energy from food.

10. Cycle and Laws
Thermodynamic Analysis is about the analysis of devices which form part of a cycle and subject the working fluid to different processes and return it unchanged back to the initial state. The laws of TD pertain to this.

11. First Law
For a substance undergoing a cyclic process, the Total Heat Input is equal to the Net Work Output. It is all about energy conservation. Heat could be removed and work added, as in Heat Pumps, so the above statement is for Heat Engines