Demand refers to the electric grid and how electricity is supplied and created when we need it. Energy cannot be stored the same way water is stored, electricity needs to be made before it can be delivered. The electricity is made at a power plant and turned into useable voltage.
Demand is instant. When you turn on a light bulb, it turns on right away. During peak hours of electricity consumption, everyone using electricity at the same time increases demand load.
Demand response is when we conserve energy. When people or communities voluntarily cut back their electricity usage.
When demand response is used, it can prevent black outs because the old power plants will not become stressed by being over worked. When less power is used it helps the environment because the power plants do not emit as many pollutants into the air.
http://science.howstuffworks.com/environmental/green-science/demand-response.htm
http://en.wikipedia.org/wiki/Demand_response
http://theenergycollective.com/petertroast/63790/demand-response-what-it-what-it-means-you