There are seven basic units of measurement that we use everyday. In SI units, These are:

Standard Mass

• Length (Meter): Equal to 1,650,763.73 wavelengths in vacuum of the orange-red line of the krypton-86 spectra
• Mass (Kilogram): Cylinder of platinum-iridium alloy kept in France and a number of copies
• Time (Second): Time for 9,192,631,770 cycles of resonance vibration of the caesium-133 atom
• Temperature (Kelvin): Absolute zero is defined as 0 Kelvin and the triple point of water as 273.16 Kelvins
• Luminosity (Candela): Intensity of a light source (Frequency 5.40x10¹⁴ Hz) that gives a radiant intensity of 1/683 watts/steradian in a given direction
• Electric Current (Ampere): Current that produces a force of 2.10^-7 Newtons per meter between a pair of infinitely long parallel wires 1 meter apart in a vacuum
• Amount of Substance (Mole): Number of elementary entities of a substance equal to the number of atoms in 0.012 kg of Carbon-12

Only the standard for mass is based upon a physical standard, and not on fundamental physical constants (Gravitational constant, Planck's constant, Speed of Light in vacuum, etc.), which means that everytime one wishes to calibrate a kilogram, it has to be physically compared with the French mass, or with some mass already calibrated using that mass. Now for the first time, the kilogram may be moving toward a new definition based on a universal constant.
A paper to be released Monday proposes redefining the unit via fixing the values of one of two well-known universal constants. The choices are either, a) Avogadro's number (measures the amount of carbon-12 atoms in 0.012 kg of that element), or b) Planck's constant (used to explain the sizes of quanta, in quantum mechanics). As reported by the Wired magazine, the change would mean the kilogram would no longer be known exactly, but would instead be determined by experiments using the chosen defined constant. A definition based on Planck's constant could mean scientists would determine a certain number of photons of light of a certain frequency would correspond to a kilogram. A definition based on Avogadro's constant could mean the kilogram would be determined by a certain number of an element's atoms.
The french mass lasted for about 100 years (first standardized in 1889, kept at the International Bureau of Weights and Measures), and has been the standard for the 20th century. Soon, perhaps, we will have a standard for the 21st century (and beyond).