This book provides a broad and complete introductions to the molecular structure, novel and anomalous properties, nonlinear excitations, soliton motions, magnetization, and biological effects of water. These subjects are described by both experimental results and theoretical analyses. These contents are very interesting and helpful to elucidate and explain the problem of "what is on earth water". This book contains the results of research by the author and plenty of scientists in recent decades. "What is water" is self-contained and unified in presentation. It may be used as an advanced textbook by graduate students and even ambitious undergraduates in Physics and Biology. It is also suitable for the researchers and engineers in Physics, Biology and water science.