Temperature and Internal Energy

Internal energy is defined as the random, disordered motion of molecules. A room temperature glass of water sitting sitting on a table has no apparent energy, either potential or kinetic. However, on the microscopic scale it is a seething mass of high speed molecules traveling at hundreds of meters per second. If the water were tossed across the room, this microscopic energy would not necessarily be changed when we superimpose an ordered large scale motion on the water as a whole.
"U" is the most common symbol used for internal energy.

Temperature is the degree of hotness or coldness of a body or environment. It is also a measure of the average kinetic energy of the particles in a sample of matter, expressed in terms of units or degrees designated on a standard scale.

The changes of state can be summarised in this slide show:
http://www.slideshare.net/meenng/changes-of-state