The official length of a meter, the metric unit of length, was decided in 1991 to be the distance from the north pole to the equator, as it runs along the French meridian near Dunkirk, divided by 10 million. The standard rule for a meter was kept as a cast metal bar made of brass. The current standard is the distance light travels in a vacuum over the course of 1/299,792,458 of a second.
The metric measurement of mass (amount of matter) is the gram, originally defined as a cubic centimeter of water at 4 degrees Celcius (its maximum density); the kilogram, therefore, is 1000 times this amount. Later, a standard weight of metal (platinum iridium) was made to represent the ideal kilogram; the standard for a gram since switching to the metal standard is one 1/1000 of this platinum iridium weight.
The measurement of temperature in the SI system is confusing for some. The official measurement of temperature is in degrees Kelvin; however, on a daily basis, most people use the Celsius measurement system instead. The Celcius system was the initial method of temperature measurement for the SI system, with the freezing point of water being defined as 0 degrees Celcius. The Kelvin system is calibrated such that 0 degrees Kelvin is equal to absolute zero.
The second is the metric measurement of time in the SI system, defined (in a nearly incomprehensible way) as the duration of 9,192,631,770 cycles of the radiation corresponding to the transition between the two electron spin levels of the ground state of the cesium 133 atom. This incredibly precise atomic standard is used to set the world's most accurate clocks -- atomic clocks -- to which all other timekeeping devices are tuned.
The three less common methods of measurement in the SI system are the mole, Ampere and candela. The mole is a measurement of substance; one mole is equal to 6.022 137 --- 10²³ of any given substance. An Ampere, named after French physicist Andre Ampere, is used to measure constant electric current. The candela is used to measure luminous (light) intensity.