A Western Disturbance is an extratropical storm originating in the Mediterranean region that brings sudden winter rain to the northwestern parts of the Indian subcontinent. It is a non-monsoonal precipitation pattern driven by the westerlies
The disturbance travels from the “western” to the eastern direction. Disturbance means an area of “disturbed” or reduced air pressure. Equilibrium exists in nature due to which the air in a region tries to normalise its pressure. 
A WD is associated with rainfall, snowfall and fog in northern India. Upon its arrival in Pakistan and northern India, clouds along with rain and snow also arrive. The moisture which WDs carry with them comes from the Mediterranean Sea and/or from the Atlantic Ocean.
whereas
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea. 
The definition of a monsoon is a seasonal, intense wind that blows from the southwest from April to October and from the northeast at other time of the year in parts of Asia, creating heavy rainstorms. An example of a monsoon is a storm that causes heavy flooding and extreme wind damage.....