The United Nations use the term "water crisis" to describe a situation observed since the 1970's. It concerns the worldwide supply of drinking water, and water pollution.
The available drinking water on Earth is not infinite. It's estimated that there are 1.1 billion people without access to drinking water and the United Nations acknowledges that there are 2.6 billion people without sufficient water for sanitation - washing your hands, etc. Potentially, that makes 3.7 billion people without enough water. That's over half of the worlds' population!
Some countries/regions particularly affected by the water crisis include Sudan, Iran and Venezuela with an estimated 22.9 million people without access to a sufficient supply of drinking water. There are many more regions affected too.
As you can learn here, water is a necessity to all known forms of life. Some species are at risk of extinction because of humans building on and using water resources in life-supporting areas such as bogs, marshes, wetlands, etc.
It's been predicted that by 2025, two thirds of the world will be without safe drinking water or water for cleaning with. If the predicted world population by then is accurate, then that's about 5.23 billion people! However, solutions are being approached. Technologies are being developed to make desalination more efficient - the process of removing salt from saltwater.
Please visit the Links page for a list of websites where you can learn more about the water crisis, and many other topics related to water.