A Type I civilization is one that controls the energy resources of an entire planet. This civilization can control the weather, prevent earthquakes, mine deep in the [home planet's] crust, and harvest the oceans. This civilization has already completed the exploration of its solar system.
A Type II civilization is one that controls the power of the sun itself. This does not mean passively harnessing solar energy; this civilization mines the sun. The energy needs of this civilization are so large that it directly consumes the power of the sun to drive its machines. This civilization will begin the colonization of local star systems.
A Type III civilization is one that controls the power of an entire galaxy. For a power source, it harnesses the power of billions of star systems. It has probably mastered Einstein's equations [or rather "UEF control"] and can manipulate space-time at will.
"The basis of this classification is rather simple: Each level is categorized on the basis of the power source that energizes the civilization."
"Type I civilizations use the power of an entire planet. Type II civilizations use the power of an entire star. Type III civilizations use the power of an entire galaxy. This classification ignores any predictions concerning the detailed nature of future civilizations (which are bound to be wrong) and instead focuses on aspects that can be reasonably understood by the laws of physics, such as energy supply."
"Ultimately, we could imagine a type `omega' civilization, which could manipulate the entire Universe (and even other universes)"
John D Barrow in `Impossibility - the limits of science and the science of limits', ISBN 0-09-977211-6
"A community which cannot or will not realise how insignificant a part of the universe it occupies is not truly civilised"
Brian Aldiss in "Non-Stop" 2000, SF Masterworks, ISBN 1-85798-998-8