The Kardashev scale is a way of classifying how technologically advanced a civilization is. The scale originally ranged from Type I to Type III, although in recent years Type 0, Type IV and Type V civilizations have been informally added.
A Kardashev Type I civilization would be one that is able to harness all the power available on a single planet. This figure has been quoted as about 10^16 Watts, though it might be much larger. To truly harness all the power of a planet would require its disassembly and reconfiguration into a gigantic solar panel much thinner and larger than the original Earth. It would also theoretically require fusing together all atomic nuclei with a mass lower than that of iron, and fissioning all atomic nuclei with a mass greater than that of iron, in order to extract the energy. The available energy would be several orders of magnitude larger than typical guesses.
Because Earth's present-day technological infrastructure has not yet consumed all the energy on this planet, it is informally known as a Type 0 civilization. Kardashev's original definition for the threshold between a Type 0 and Type I civilization was one with an available power level of 4 x 10^12 Watts.
A Kardashev Type II civilization would harness all the power available from a single star. This figure is around 10^26 Watts, but as in the last case, it has likely been underestimated. Although one might think that a Dyson sphere, a solar panel encasing the entire sun, would be sufficient to harness all the sun's energy, to truly tap everything would require disassembling the sun and creating a lattice of mini-reactors that fuse together nuclei in the most efficient possible way. Although figures of thousands of years in the future are sometimes given for the transformation of humanity to a Type II civilization, nanotechnology and artificial intelligence might make this possible much sooner than previously expected.
A Kardashev Type III civilization would harness all the power in a single galaxy. This has been estimated at around 10^36 Watts, although Kardashev's original definition was 4 x 10^37 Watts. With the power of an entire galaxy at its disposal, a civilization would likely be able to come up with radical new power sources, perhaps extracting energy from the vacuum itself. With self-replicating space probes carrying the full set of capabilities necessary to kickstart a stellar civilization, galactic colonization could happen almost as quickly as the speed of light travels.
A Kardashev Type IV civilization would be one that consumes the power of an entire galactic supercluster. Kardashev Type V, of course, would be a civilization that occupies the entire universe.