1. POWERS OF TEN “power of 10 ” scientific notation:
means 10 raised to some number; e.g.: = 10  = 10  10  10  10  n = n factors of 10; n = “exponent”
scientific notation:
express numbers as: a  10n , where a is a number “of order 1” (i.e. between 1 and 10)
negative exponent: n means 1/(10n)
to multiply two powers, add their exponents:  105 = 107
thousand = 103 , million = 106
common prefixes used to denote powers of ten:
kilo (k) = 103 ,
Mega (M) = 106 ,
Giga (G) = 109
Tera (T) = 1012
milli (m) = 10-3 ,
micro () = 10-6
nano (n) = 10-9
pico (p) = 10-12

2. Units
“Metric units”
apart from few exceptions, “metric units”, or rather “SI units” are used throughout the world; (“SI” stands for “Système International”, maintained by the “Societé Internationale des Poids et Mesures”, = international society of weights and measures, with seat in Sèvres near Paris)
basic units are, e.g.: meter for distance, kilogram for mass, second for time;
meter originally defined as 1/(4  107)  length of Earth's meridian; now defined in terms of speed of light;
some common distance units:
kilometer, km = 103 m,
centimeter, cm = 10-2 m,
millimeter, mm = 10-3 m,
micrometer, µm = 10-6 m,
nanometer, nm, 10-9 m
“metric” vs “English” distance units:
1 km  0.62 mi, 1mi  1.6 km,
1 m  3.3 ft  39 in,
1cm  0.39 in, 1 in = 2.54 cm