Heat transfer exchanges energy between two systems. This transfer can be between solid objects, liquids, gases or a combination of state types. There are three types of heat transfer: conduction, convection and radiation.
Conduction is the direct heat transfer of two objects in contact with each other. Convection is transfer due to flow of a liquid or gas across another object; the higher the speed of the liquid or gas, the higher the amount of heat transfer. Radiation is the transfer of heat energy in the form of electromagnetic waves.
Anything that has mass has energy. The molecules of objects vibrate in relationship to the heat they contain; the more heat, the more vibration. This is easy to see when water cools to the point of freezing. As the water cools, the energy contained in the water molecules decreases, slowing them down. At zero degrees Celsius, molecules reach a point that they are no longer able to stay in liquid form and become ice. At the boiling point of water, 100 degrees Celsius, water has obtained enough heat transfer to form a gas, seen as steam. Absolute zero is the lower limit of heat. It predicts that once this limit is reached, molecular motion of a crystal stops entirely.