Water harvesting is what you collect rain water directly to use in your garden or at home for cooking, showering, and laundry. This can be done by collecting the water that comes from your rain gutters into a plastic container. This is a good backup water system if something were to happen where the city water stopped running. You can also reduce your water bill in the process.
But is it safe to drink? As rainwater is being collect from your roof, it could pick up contaminants such as bird poop, leaves, or bugs. You can use a mesh to keep these contaminants from going into your water barrel. The barrel itself should have a cover so that mosquitoes do not go in and laying eggs.
You may want to look into water purification solutions if you are going to use rain water for drinking. For short term use you will want to boil the water. For more information on making rainwater drinkable, click here for an article from Rainwater.co.va.