Reversible Process: It is a process which can be made to proceed in two opposite directions with same ease, so that the system and surroundings pass through exactly the same intermediate state as in the direct process.
e.g., An ideal gas allowed to expand slowly and then compressed slowly so as to reach its initial state.
Irreversible Process: It is a process which can't be made to proceed in the reverse direction with the same ease and the system does not pass through the same intermediate states as in direct processes.
e.g., Decay of organic matter, rusting of iron.