In broad terms, Thermodynamics is a discipline of study that deals with how the energy of
one form gets converted to another. It gives us the relation between variables like heat, work,
temperature, and energy that define a physical system.
The first law dictates that the energy of the universe remains constant; it only gets converted from one form to the other.
The second law tells us that any spontaneous process occurs only in the direction of increasing randomness or disorderliness i.e., entropy. This law clearly explains why it is impossible to create a machine with 100% efficiency as it cannot fully convert heat into mechanical energy.
Thermodynamics undoubtedly lays down the fundamentals in science but has many limitations. For example, it cannot directly determine the equation of state of a system of particles like,
PV = nRT
for ideal gases1.
It can, however, give a relation between variables and predict the average value of the change between them. For example, thermodynamics provides the relation between average difference between the specific heat of a substance under constant pressure (Cp) and constant volume (Cv) to universal gas constant (R) as,
Cp - Cv = nR
where n is number of moles of gas but cannot determine the absolute value of specific heat of a material under constant volume or pressure. Similarly, it cannot determine the absolute value of entropy of a system but the average change of entropy during a process.
Meanwhile, the third law of thermodynamics states that the entropy of a pure substance in a perfect crystalline state at zero temperature is zero.
S = k loge(W)
To know the exact value of these variables, one needs to resort to the concepts of Statistical Thermodynamics.
1 The terms have usual significance