H2O, commonly known as water, is a vital chemical compound composed of two hydrogen atoms and one oxygen atom. It is essential for all known forms of life and covers about 71% of the Earth's surface, primarily in oceans, rivers, and lakes. Water exists in three states: liquid, solid (ice), and gas (water vapor). It plays a crucial role in various biological processes, including hydration, nutrient transport, and temperature regulation. Additionally, H2O is a universal solvent, making it indispensable in chemical reactions and environmental systems.