Thermodynamics is the study of heat and its interaction with matter.
Thermodynamics works with the measurable properties of a system such as its volume, pressure, temperature etc. and aims at predicting the spontaniety (or non-spontaniety) of a process that a system can undergo.
Thermodynamics predicts the spontaneity by taking into account the changes in the different functions of state of the system as a result of the given process. The functions of state of a system are the parameters of the system that depend only upon its state at the specified time, and not on the previous processes through which that state has been achieved.
Thermodynamics aids us in predicting the amount of work that can be extracted from a system by providing it with a certain amount of energy.
Thermodynamics works with the help of concepts of system and surroundings and with functions of state of a system like Internal Energy, Enthalpy, Entropy, Free Energy etc. By measuring the changes in these functions for a process, one can tell whether the process is feasible ordinarily or not.
The significance of these parameters lies in the manner of their definition. The actual laws at work here are important laws of thermodynamics which are fundamental laws of nature defined in terms of the above mentioned quantities.