The metric unit of length is the meter, defined as the length over which light travels in a vacuum in 1/299,792,458 seconds. This definition was adopted by the CGPM in 1983, superceding the previous definition, which was based upon a wavelength of krypton-86 radiation. The purpose of these definitions was to provide an exact, repeatable alternative to the official meter bar, which was susceptible to wear. More commonly, the meter is defined in relation to other units of measure, such as the base unit of length in the U.S. Customary System, the foot. One meter equals approximately 3.28 feet.
The base unit of mass is the kilogram. This unit was originally defined as the mass of a cubic decimeter of water, but in 1889, the CGPM declared a specially-made platinum-iridium cylinder the new official standard. This prototype remains in the International Bureau of Weights and Measures near Paris, France, to this day. This unit is distinct from the unit of weight, which is the newton, although the two terms are often confused. One kilogram is approximately 2.20 pounds on the Earth's surface.
The standard unit of time is the same in both the metric and the U.S. Customary System --- the second. Although originally defined as a fraction of a day, this definition was superseded in 1967 because of variations in the Earth's rotation. It is now defined as the length of time it takes for the cesium-133 atom to give off 9,192,631,770 cycles of radiation produced by the transition between two hyperfine levels, or very small sub-levels within the atom's energy level.
The ampere (A) is the metric unit of electric current, as established by the CGPM in 1948. This is the amount of current that produces 2 x 10^-7 Newtons of force per meter of length between two wires placed one meter apart in a vacuum. This unit is also commonly called an "amp." Amps are the only commonly used measure of current in the U.S. and Europe.
The metric base unit for temperature is the Kelvin, which is defined as 1/273.16 of the temperature of the triple point of water. The triple point is the temperature at which water exists simultaneously as a solid, liquid and gas --- exactly 273.16 Kelvin, according to the CGPM. A temperature of 0 Kelvin is theorized to be the point at which all atomic motion, except for quantum mechanical motion, ceases. Thus, it is called absolute zero. At this point, absolutely no heat energy remains. Kelvin degrees are equivalent to degrees Celsius; the only difference lying in the starting point of the scale. Zero degrees Celsius equals 273.15 Kelvin.
The metric unit of quantity is the mole, which is the amount of a substance equal to the number of atoms in 0.012 kilograms of carbon-12. This quantity is equal to Avogadro's number, which is 6.022141527 * 10^23. Scientists often use this unit when describing a quantity of atoms or molecules. It is especially useful for performing stoichiometric conversions, as one mole of any element has a mass in grams equal to its atomic mass.
The candela is the metric base unit of luminous intensity. It was originally based upon the light of a standard candle, then the radiance of a blackbody. But when the latter proved too difficult to reproduce, it was redefined again in 1979. This latter definition established the value of one candela as the intensity of a light emitting a frequency of 540 x 10^12 hertz, and which gives off 1/683 watts per steradian.