Rainwater harvesting can be as simple as attaching a water butt to the down pipe from your gutters and using the water you collect to water the garden or wash the car.
At the other end of the scale, it can be a pumped system that uses rainwater for toilet flushing and clothes washing, as well as outdoor uses. These systems might be gravity fed or use pumps.
Water from your roof is channelled via your drainpipe to a storage tank. If you want to use it in your home the water will pass through a filter before being stored and used. More sophisticated installations need a control system to monitor water levels and a pump to get the water to where you want it.
A system that feeds the home will need mains back up that cuts in automatically when the storage tank runs dry. The plumbing from the system must be kept separate from the mains system and labelled. The storage tank can be at ground level, but it is preferable to bury it, both because it looks better, and to protect it against frost.
Rainwater should not be used as drinking water.