Great site: Going Green Consulting

"Graywater is the water that goes down the drains from your washing machine, bathroom sinks and showers. When you stop to think about it, the average top-loading washing machine will use between 25 and 35 gallons of water per load. Just multiple that out by how many loads you do a week, and then a month, which you can see will quickly add up to big numbers. Now imagine your shower and how many gallons that is… A lot of wasted water that is going down the drain.

Graywater harvesting is the capture, filtering, and reuse of this water. The most common use of this graywater is for irrigation purposes. Another use that is starting to catch on is the shower/sink to toilet system. Is is basically capturing water from your bathroom shower and/or sink to be used for your toilet tank."