A process is said to be reversible when the system and the surroundings return to their original state from the final state, with no changes anywhere else in the universe.  Some of the conditions to be met for a reversible reaction are

The process should be at an extremely slow rate

• There should not be any unbalanced forces
• The system should be in thermal equilibrium
• The system should be in chemical equilibrium

Example – Slow compression and expansion of spring

A process is said to be irreversible when the system and the surroundings cannot return to their original state from the final state.

Some of the reasons for an irreversible reaction are

• The process changes the system to a non-equilibrium state.
• Most of the process involve friction, viscosity and other dissipative effects

Example – Rusting of iron