Standard Unit of Measurement
To maintain uniformity in measurement, standard units of measurement were introduced in different parts of the world at different places. It  have a fixed quantity and, therefore, do not vary from person to person and place to place. For example, the metric system, created by the French in 1790, is a standard set of units. Adopting standard units of measurement solves only half the problem. People in different countries may be using different sets of standard units for measurement. For example, gram and pound set of units.  The adoption of SI units has made it easier for scientists of different countries to communicate their results to one another.

SI Unit (International System of Units): The SI system of units was adopted in 1960 by the  General Conference of Weights and Measures. SI is the short form of system of International Units (in French).
It is the most widely used system of measurement in the world. It is based on MKS (Meter-Kilogram-Second) System. Length, weight and time are considered as basic quantities for measurement and other quantities are derived from them. As per SI system; the unit of length is meter, unit of weight is kilogram and the unit of time is second.
Depending on the size of the object we wish to measure, we have to choose an  appropriate unit. For example, we use metres to measure the length of  a piece of cloth, kilometres to measure the distance from one place to another, millimetres to measure the thickness of the hair, and so on. Units of length can be interconverted by multiplying or dividing their values by 10, 100, 1000, and so on.
A commonly used unit for measuring large distances is the kilometre (represented as km). Commonly used units for measuring small distances are the centimetre (cm) and millimetre (mm).
1000 m = 1 km    
100 cm = 1m  
1000 mm = 1 m    

For measuring lengths smaller than the metre, its sub-multiples are used. For lengths greater than the metre, its multiples are used , Those are given in following table:-
Relation between some units :
10 mm =1 cm, 10 cm = 1 dm, 10 dm = 1 m; 
10m = 1 dam, 10 dam = 1 hm, 10 hm = 1 km