All measurements made in any science must be reported in some unit: for example, 1.63 meters long, 12.9 degrees Kelvin, etc. In the sciences, these units are made in the metric system. The common base units of the metric system are shown in the table below

Base quantity	Base unit	Abbreviation
Length	meter	m
Mass	gram	g
Temperature	degree Kelvin	K
Time	second	s
Amount	mole	mol
Electric current	ampere	A

From the above units, a wide variety of derived units can be made. For example, volume can be expressed in cubic meters, m³. Density can be expressed in terms of grams/m³, and so on.

The metric system also uses a set of prefixes to make writing very small or very large quantities easier. Common prefixes are shown below

Prefix	Abbreviation	Factor
exa	E	1018
peta	P	1015
tera	T	1012
giga	G	109
mega	M	106
kilo	k	103
hecto	h	102
deca	da	101
deci	d	10-1
centi	c	10-2
milli	m	10-3
micro	m	10-6
nano	n	10-9
pico	p	10-12
femto	f	10-15

For example, if you had something with a mass of 0.00100 grams, you could use the prefix "milli" to express the mass as 1 milligram, or 1 mg.

Example: How many meters are in 1.45 km?

Solution: The k prefix means "kilo", or 10³. Thus, 1.45 km = 1.45*10³ meters

Information provided by: http://learn.chem.vt.edu