Water storage tanks answer a question as basic as human civilization itself, which is how to store water in order to deal with times of scarcity. This is especially true for areas away from practical reserviors, such as rivers or natural springwells, and for areas prone to drought. Most towns have a community water tank, which is the large and globular tower you may see on a high hill. It is filled with purified drinking water, and it is elevated both to keep it cool and to provide water pressure due to gravity.
For a home in a remote area, you must have your own reservior. This might be a well, like in the old days, but many ground springs have run dry due to commercial drilling and tapping. Groundwater may also contain chemical polutants. For better or worse, you must have a fresh supply, which means a commercial tank with purified drinking water. There are many models to choose from, not to mention a considerable price range.
Tanks can range from above ground to below ground. Tanks above ground provide water pressure, but water must be put into them mechanically, and they may also heat in the summer sun. Ground tanks stay cool if they are burried deep, and will not freeze except at the most northerly latitudes. Also, ground tanks can collect rainwater. In this case, it is called a sistern. Rainwater is a useful resource, but it needs to be filtered and chemicals typically must be added to prevent any bacterial growth.
Sometimes your needs are temporary, such as having run out of water on a farm. In this case is possible to turn to water tank rental, or having someone bring you hundreds of gallons of water by truck to your place. In some cases, the water tank is the entire trailer, and the truck just leaves the trailer. Other times, a plastic or metal tank is brought on a flatbed and left where you specify.
Hot water tanks are designed to be heated or even be a solar collecting agent. Many people install solar water heaters on their roofs, and the hot water is circulated and stored in the tank.