In thermodynamics we define the state of a substance in terms of the various properties we can attribute to it.
Internal Energy Q
A process is said to be reversible when the successive states of the process are infinitesimally close to equilibrium States. i.e. the process is quasi-equilibrium.
With a reversible process it is possible to restore the system to its original state without needing an external agent or changing its surroundings.Reversible processes are an abstraction that aids the analysis of real processes.
A reversible process is a standard of comparison for an actual system. Truly reversible thermal processes would require an infinite amount of time for completion.
If a system is in equilibrium it state variable do not change with time
A process is called a quasi-equilibrium process if the intermediate steps in the process are all infinitesimaly close to equilibrium. In this way we can characterize the intermediate states of the process using state variables.
When a process is quasi-equilibrium we can plot a graph of P against V the path of the process. Areas under the graph then represent the work since all the variable used to characterize the substance's intermediate states have well defined values.
Most of the process you will encounter will be quasi-equilibrium processes
In practice, all Natural processes are Irreversibl processes. The path of an irreversible process is indeterminate and cannot be drawn on a thermodynamic diagram. (We use a hashed line to indicate the path because the intermediate states are in non-equilibrium.)
The Entropy of the universe always increases during an irreversible process. It is always possible to restore an irreversible process to its original state by a reversible process, but the Entropy of the universe can never be restored. An irreversible process always requires an external agent to restore it to its original state.
Examples of Irreversible Processes include:
Your House Getting Dirty