It means capturing rain where it falls or capturing the run off in your own village or town. And taking measures to keep that water clean by not allowing polluting activities to take place in the catchment.

Therefore, water harvesting can be undertaken through a variety of ways

• Capturing runoff from rooftops
• Capturing runoff from local catchments
• Capturing seasonal floodwaters from local streams
• Conserving water through watershed management

These techniques can serve the following the following purposes:

• Provide drinking water
• Provide irrigation water
• Increase groundwater recharge
• Reduce stormwater discharges, urban floods and overloading of sewage treatment plants
• Reduce seawater ingress in coastal areas.

In general, water harvesting is the activity of direct collection of rainwater. The rainwater collected can be stored for direct use or can be recharged into the groundwater. Rain is the first form of water that we know in the hydrological cycle, hence is a primary source of water for us. Rivers, lakes and groundwater are all secondary sources of water. In present times, we depend entirely on such secondary sources of water. In the process, it is forgotten that rain is the ultimate source that feeds all these secondary sources and remain ignorant of its value. Water harvesting means to understand the value of rain, and to make optimum use of the rainwater at the place where it falls.

http://www.rainwaterharvesting.org/whatiswh.htm