What is a fundamental unit?

Answer 1

A fundamental unit is a unit that cannot be broken down any further. Other units are based on derivatives of these units.

http://en.wikipedia.org/wiki/Fundamental_unit

In most countries, the SI base unit system is used

http://en.wikipedia.org/wiki/SI_base_unit

Things like distance and time cannot be broken down further, so base units are used (metre and second respectively).

An example of a derived unit is km/h (kilometres per hour, often seen kmph), where kilometres are based on 1000 metres, and hours being 3600 seconds (60 x 60), where speed is equal to distance divided by time.