What is the difference between derived units and base units?

Example:
1 km (derived unit)
1000 meter (base unit)
100000 (centimeter)

1.  Unit of measurement is a numerical value that identifies the magnitude of a physical quantity. Hence, there were a lot measurement that was used pre-modern era. And now, Internation System of Units is used.
2.  System of measurement is a range, assortment and group of units of measurement in a single quantity to be measured which entirely related to each other.
3.Tools of measurement are devices that construes a quantity or a series of system measurement. It an object that relies on being valid and reliable, used in many certain areas like physical quantities, psychological constructs or other entities that can be measured and has a system of measurement.