When man started to wonder about the physical phenomena thousands of years ago he realized that in order to express an idea about the size of something or its weight or the space it occupied, he needed to compare it to something he knew that he needed to compare it to something everyone knew. The first standards of length were the palm of the hand, the foot, the cubit (which is the distance from the elbow to the tip of the middle finger), but all of this standards were changeable, it is unlikely to find two persons who have their palms of their hands the same size.
So later on man managed to find standards that weren't changeable, standards that remained the same all of the time, and that were universal. This is when Unit Measurement Systems were born. Nowdays the two most common Unit Measurement Systems are the International System, usually refereed to as SI; and the Metric System.
The International System is based on the MKS (meter-kilogram-second) system. In a conference held in 1960 standards for six base units and for two additional were defined. In 1975 a seventh base unit was added: the mole. The following table shows the seven base units and the two supplementary ones are listed in another table below, the symbol of each unit is universal, this means it is the same in all languages.