The International System of Units (SI) constitutes a catalogue of units in which the measurements of different magnitudes should be expressed.
If we say, for example, that the temperature is 25 ºC we are not using the unit for temperature proposed in the SI and that is because in normal language there is no reason to use SI units.
If we wanted to express this temperature in SI units we would have to say that it was 298 K (where K means kelvin). Multiples and submultiples of the units are commonly used as it does not make much sense to measure the mass of a sheet of paper in kg.
If you click here you will be able to see the names and the symbols of the multiples and submultiples of the physical units.