Water is unique in occurring in all three states of matter, gas, liquid, and solid , within a narrow range of temperatures. The freezing point of water is the temperature at which it changes from a liquid to a solid. The boiling point of water is the temperature at which it changes from a liquid to a gas. Energy is required to overcome these forces and allow a change of state. The high freezing and boiling points of water are the result of having to overcome both these forces and the hydrogen bonds to change the state of water.
Heat is the energy of moving molecules.
A calorie is the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius. In the solid state we have a rigid form and structure of matter and it does not flow. The dominant molecular motion is vibration. In the liquid state molecules are unstructured and take the shape of their container. Many of the van der Waals' forces found in the solid state have been overcome in the liquid state.