Thermodynamics, reversible process I understand that a reversible process is a quasistatic process but I am not able to understand what is reversible in a reversible process and what is irreversible in a irreversible process?
 A: It sounds a bit pithy, but it's the process which is irreversible. 
Irreversible processes are spontaneous. This means that they naturally happen on their own in nature. Examples of irreversible/spontaneous process are heat transfer from a hot body to a cold body and diffusion from high concentration to low concentration. If we label the initial state as A and the final state as B, the fact that the process is irreversible just means that the process A$\rightarrow$B will never run backwards. It is possible to return the system to state A, we'd just need to use a different process. If we consider the heat transfer example, we could return the system to its original state by transferring heat to the previously-hot body (from an even hotter body) and from the previously-cold body (from a colder body). This would return the system to its original state, and it would use heat transfer, but it would not be a direct reversal of the process A$\rightarrow$B. The reverse process, B$\rightarrow$A, would involve heat spontaneously flow from the cold body to the hot body; this is impossible and doesn't naturally occur.
