The definition for supply includes the complete supply curve depicting how much supply of a product or service exists at various prices. The quantity supplied, however, represents the amount of supply for a product or service that is available at a specific price point along the supply curve.
While those speaking of supply in general are usually referencing the entire supply curve, the quantity supplied is the actual amount of product available. The quantity supplied changes according to the change in price, so it helps the manufacturer determine just how much can be earned at different price points.
As prices rise for a product or service, the quantity provided tends to rise as well. This is due to companies chasing the profits available with higher prices, producing as much of the product as the market will allow. When prices are lowered, the quantity supplied decreases for the opposite reason.
Because the supply curve deals entirely with the intersection of prices and quantity supplied, it can shift entirely on a graph when affected by other factors of production. These factors include technology changes, changes in production costs and changes in the prices of related goods. When this occurs, the supply curve shifts to the right or left. A right shift indicates a positive change in the quantity supplied at all price levels, while a leftward shift shows that supply has lessened, reducing the quantity at all price levels.