All units in science are derived from seven base units:

Mass	kilogram	kg
Distance	metre	m
Time	second	s
Current	ampere	A
Amount	mole	mol
Temperature	Kelvin	K
Light Intensity	candela	cd

There are many other units that we use, but all of these are derived by multiplication or division of some combinations of the base units.

You can think of it like letters and words. We have 26 letters in the alphabet but we have thousands of words in our language. Here are some of the derived units:

Quantity	Unit	Symbol	Base unit equivalent
Velocity	metre per second	ms-1	ms-1
Acceleration	metre per second squared	ms-2	ms-2
Force	Newton	N	kg ms-2
Work or Energy	joule	J	kg m2s-2
Power	watt	W	kg m2s-3
Pressure	Pascal	Pa	kg m-1s-2
Frequency	hertz	Hz	s-1
Charge	coulomb	C	A s

Now you have units, you often need to group these into larger or smaller numbers to make them more manageable. For example, you don't say that you are going to see someone who lives 100,000 m away from you, you say they live 100 km away from you.

Here a quick list of the common quantities used:

Name	Symbol	Scaling factor	Common example
tera	T	1012 1,000,000,000,000	Large computer hardrives can be terabytes in size.
giga	G	109 1,000,000,000	Computer memories are measured in gigabytes.
mega	M	106 1,000,000	A power station may have an output of 600 MW (megawatts).
kilo	k	103 1,000	Mass is often measured in kilogrammes (i.e. 1000 grammes).
deci	d	10-1 0.1	Fluids are sometimes measured in decilitres (i.e. 0.1 litre).
centi	c	10-2 0.01	Distances are measured in centimetres (i.e. 100th of a metre).
milli	m	10-3 0.001	Time is sometimes measured in milliseconds.
micro	µ	10-6 1,000,000th	micrometres are often used to measure wavelengths of electromagnetic waves.
nano	n	10-9	nanometres are used to measure atomic spacing.
pico	p	10-12	picometres used to measure atomic radii.