Metric System of Measurement
(Correctly called "SI")
The metric system is a system of measuring. It has three main units:
m  the meter for length  
kg  the kilogram for mass  
s  the second for time 
we can measure nearly everything in the world!
Examples:
Meter
The length of this guitar is about 1 meter: 
When unfolded this ruler measures 2 meters: 

Kilogram
This gold bar has a

A dictionary also has a mass of about 1 kilogram. 
Second
1 second is about as long as it takes to say "one thousand and one" 
Larger or Smaller
But what if we want to talk about really big or really small things?
Answer: we can use Metric Number Prefixes
 like "kilo" (a thousand)
 and "milli" (one thousandth)
 and so on
Example: something that is 1,000 meters is a "kilometer"
Something that is one thousandth of a second is a "millisecond"
In fact the kilogram already uses this method, as it's a thousand grams, a kilogram.
So one thousandth (1/1000) of a kilogram is simply a "gram"
Here is a quick summary of the special prefixes:
Large Numbers
Name  deca  hecto  kilo  mega  giga  tera  peta  exa  zetta  yotta 

Symbol  da  h  k  M  G  T  P  E  Z  Y 
Factor  10^{1}  10^{2}  10^{3}  10^{6}  10^{9}  10^{12}  10^{15}  10^{18}  10^{21}  10^{24} 
Example: A million liters is called a megaliter and abbreviated ML
Small Numbers
Name  deci  centi  milli  micro  nano  pico  femto  atto  zepto  yocto 

Symbol  d  c  m  µ  n  p  f  a  z  y 
Factor  10^{1}  10^{2}  10^{3}  10^{6}  10^{9}  10^{12}  10^{15}  10^{18}  10^{21}  10^{24} 
Example: A thousandth of a meter is called a millimeter and abbreviated mm
Making Other Units
We can also combine the meter, kilogram and second to make new Units of Measurement!
Example: Speed
Speed is how far something moves over a period of time
So it can be measured in meters per second
It means: How many meters does something travel in one second
We can write it as meters/second, or simply m/s
Here are a few common units that are based on the meter, kilogram and second:
Area
Square Meter
Area is length by length, so the basic unit of area is a square that is 1 meter on each side.
The Unit is meters × meters, which is written m^{2} (square meters).
Volume:
Cubic Meter
Volume is length by length by length, so the basic unit of volume is a cube that is 1 meter on each side.
The Unit is meters × meters × meters, which is written
m^{3} (cubic meters).
Liter
So, a cube that is 1 meter on each side is a cubic meter (m^{3}) ...
... and that is also equal to 1,000 liters.
1 m^{3} = 1,000 Liters
Liter is abbreviated L (some people use lowercase l, but that looks too much like 1).
So a liter is actually onethousandth of a cubic meter.
1 Liter = ^{1}/_{1000} m^{3}
Another way of thinking about a liter is:
 A box that is 0.1 meters (10 cm) on each side,
 One square meter that is millimeter thick.
Time
Hour
An hour is 60 minutes, and a minute is 60 seconds, so an hour is:
 60 × 60 = 3,600 seconds
Day
A day is 24 hours so:
 1 day = 24 × 60 × 60 = 86,400 second
Speed
Speed in meters per second (m/s)
This is a combination of two units (meters and seconds) to make a new one (m/s).
If something is traveling at 1 m/s it moves 1 meter every second.
Speed in kilometers per hour (km/h)
A kilometer has 1000 meters, and an hour has 3600 seconds, so a kilometer per hour is:
 1000 / 3600 = 1/3.6 = 0.277... m/s
How did I know to make it 1000/3600, and not 3600/1000 (the other way around)? Read how to Safely Convert From One Unit to Another.
Acceleration
Acceleration is how fast Speed changes.
If something accelerated from a Speed of 5 m/s (5 meter per second) to 6 m/s (6 meters per second) in just one second, it has accelerated by 1 meter per second per second!
That is two lots of "per second" and is written m/s^{2}:
Force
Force is usually measured in the Unit of Newtons, an important measurement in Physics and Engineering.
A Newton is how much force it takes to make 1 kg accelerate at 1 m/s^{2}.
Which is written as 1 kg · m / s^{2} (one kilogrammeter per secondsquared).
So force is actually based on the meter, kilogram and second.
So force is a combination of the three basic units.
SI
The Metric System had its beginnings back in 1670 by a mathematician called Gabriel Mouton.
The modern version, (since 1960) is correctly called "International System of Units" or "SI" (from the French "Système International").
So we should really call it "SI", but mostly people just call it "Metric".
A few special units are also needed to complete the SI System:
 ampere for electric current,
 kelvin for temperature,
 mole for the amount of substance, and
 candela for luminous intensity
So the complete list is:
Quantity  Name  Symbol 

Length  meter  m 
Mass  kilogram  kg 
Time  second  s 
Electrical Current  ampere  A 
Temperature  kelvin  K 
Amount of substance  mole  mol 
Luminous intensity  candela  cd 