What is a demand curve?

The Demand Curve is the quantity and quality of need for a service or a product.

For example, if Car part X is sold on September in the Market is 100 pieces, while in October 200 Pieces and November 250 pieces that means that the demand curve on this Part X is increasing and if all the surrounding circumstances are stable and the same so you can predict the approximate number of pieces to be sold next Month which is of course must be above 250 pieces