How everything started...
About 100 years ago, in the year 1899, Max Planck (PDF-File, 99kB) discovered a new physical constant, the so-called Planck constant, abbreviated by the letter h, which is also called a quantum of action.
Planck discovered h while working on the emission of radiation of a warm black body. After all attempts failed to explain the radiation spectrum by the known laws of thermodynamics or electrodynamics, he proposed a very peculiar equation. It says that the energy radiated by a "black" body is not radiated continuously but, and this was revolutionary, in small "packages", which he called quanta. The energy of these quanta is the product of the frequency ν (Greek nu) of the radiation and the Planck constant.
E = h × ν
The frequency ν is a quantity, which is measured in units "per second" or 1/s. The quantum of action h is measured in the units of Joule × seconds (Js), which is the work (unit J) times the duration (unit s) of its action, which is the reason for the name of this constant. It has a very small value of h = 6.626*10-34 Js.
If one multiplies the frequency ν of a radiation by its propagation velocity v, one gets its wavelength λ as: λ = ν × v. One of the consequences of the equation is that the radiation with a short wavelength has a higher energy than the one with a longer wavelength.