Don't waste all that free water!
This is is just going to be a quick post to reference the formula for calculating how much water can be obtained from a roof (it is A LOT, by the way).
Formula for calculating rain collection - from Gaia's Garden.
For those who don't like to do a lot of math, here is a quick reference for rain water collection.
The basics of rain harvesting from your roof is quite simple. First you need gutters. Then you need somewhere to store the water. That is it. To make things more efficient, your storage tank is situated above (higher) than the location you need it, so gravity will take it to where you need it.
Traditional style of rain barrel.. what most people think of.
Modern Rain Barrels come in a variety of shapes and sizes.
More styles of rain barrels.
The basic function of a rain barrel.
Another diagram of a rain barrel.
Here is a more detailed diagram of a rain water collection system. If you are going to reuse the water in your own home, there are a few additional components to think about. First is a pump. If you are fighting against gravity, this is obviously needed. Second, a filtration system is needed if you will be drinking the water. Third (or maybe considered part of the filtration system) is a way to separate the first water off the roof which carries roof dust, bird droppings, and other particles that you don't want in your water storage tanks. These contraptions go by various names such as "roof washer", "flush diverter", or "first flush diverter".
A "Roof Washer"
More detailed photo of the "Roof Washer"
Note the ball that floats as the first roof water flows past.
As the water rises (and becomes more and more clean) the ball will eventually form a seal, and the clean rain water will be diverted to the storage tank.
Another more basic design.
Yet another variation of a first flush diverter.
Here is a link to a PDF from the University of Georgia Extension Agency on Home Rainwater Harvesting. It is a pretty good article.