"Systeme International", or SI Units, is a standardised system of measurement based on internationally agreed definitions.
It has 7 Base Units, from which all other units are derived:
Metre (m) - Length
Kilogram (Kg) - Mass
Second (s) - Time
Kelvin (°K) - temperature
Ampere (A) - Current
Candela (Cd) - Luminous Intensity
Mole (mol) - Amount of a substance
All the base units have standard definition, for example, the metre is the distance travelled by light in a vacuum in seconds.
Other units are derived from the base units. For example:
Velocity (ms-1)
Acceleration (ms-2)
Density (Kgm-2)
Some of the derived units also have their own special unit names. For example:
Coulomb (C) - (As) Charge
Pascal (Pa) - (Kgm-1s-2) Pressure
Ohms (Ω) - (Kgm2s-3A-2) Resistence
Prefixes
In order to make working with large or small numbers more convenient, a system of prefixes is used, where the unit is multiplied by a certain power of ten:
Yotta (Y) - 1024
Zetta (Z) - 1021
Exa (E) - 1018
Peta (P) - 1015
Tera (T) - 1012
Giga (G) - 109
Mega (M) - 106
Kilo (k) - 103
Milli (m) - 10-3
Micro (μ) - 10-6
Nano (n) - 10-9
Pico (p) - 10-12
Femto (f) - 10-15
Atto (a) - 10-18
Zepto (z) - 10-21
Yocto (y) - 10-24
In addition, these prefixes are also used:
Hecto (h) - 102
Deca (da) - 101
Deci (d) - 10--1
Centi (c) - 10-2
For example, 0.005 A can be written as 5 mA, and 6900000000000000000000000 m can be written as 6.9 Ym.
seconds.