demand

supply/demand

demand
a consumers desire
The need to have certain things

supply
how much is available
amount of goods that can be sold

demand falls
supply rises

prices fall

prices rise

demand rises
supply falls

The cost of a product is determined by
supply and demand

http://en.wikipedia.org/wiki/Supply_and_demand

If demand increases and supply remains unchanged, a shortage occurs, leading to a higher equilibrium price.
If demand decreases and supply remains unchanged, a surplus occurs, leading to a lower equilibrium price.
If demand remains unchanged and supply increases, a surplus occurs, leading to a lower equilibrium price.
If demand remains unchanged and supply decreases, a shortage occurs, leading to a higher equilibrium price.