The Planck Constant (h) is 6.626068 × 10-34 m2 kg/s. The relation of a Photon's energy to frequency is
E = h × f
(E being energy, f being Frequency).
The Reduced Planck Constant (or Dirac Constant, abbreviated ℏ) is h/2π, which is convenient because many equations incorporate this expression.
A set of units based upon fundamental constants, including the Planck constant determine some minimal scales regarding current physical laws: Planck Length, Planck Mass, and Planck Time. For example, a black hole of the Planck mass (a Planck Hole) is considered as small as they can be.
tP ≡ √(ℏG/c5) mP ≡ √(ℏc/G) lP ≡ √(ℏG/c3)