A cistern is a tank which holds water. They come in many shapes and sizes, but the most common is probably a simple cylindrical tank with a lid. Cisterns are often used to catch and store rainwater. When it rains, the lid is left off, and later the lid is put back to prevent rubbish such as drifting tree leaves from getting in.
Some cisterns consist of a hole dug into the ground, with a lining of stone and mortar or some other material. They look like a well at first glance, but while the walls of a well are porous, allowing groundwater to seep through, the walls of a cistern are water-tight, to keep water in, and a cistern is generally much shallower than a well. Other cisterns sit on the ground, or on some raised structure.
Water from a cistern may be used for drinking, cooking and for washing if it is clean enough; otherwise it can still be used for watering plants and animals, or for flushing toilets.