A shift of a supply curve is caused by a change in an underlying condition of supply, such as the cost of raw materials, rather than the price of the good or service.
The following supply schedule shows the original supply against price, with two further columns - S1 indicates an increase in supply of 400 units at each price, and S2 indicates a reduction in supply of 400 units.
Taking data from the supply schedule we can identify S1 as an increase in supply - where the supply curve shifts to the right.
Assuming price is constant, a shift to the right (at S1) is an increase in supply which could be caused by;
A shift of a supply curve to the left (at S2) is a decrease in supply. Assuming the price is constant, a shift in supply to the left could be caused by: