The metre is the standard of length, which is divided into 100 centimetres,
also into 1000 millimetres. 1000 metres is the same as 1 kilometre.
And we can also see that 10 millimetres is the same as 1 centimetre.
These distances can all be abbreviated to 1 or 2 letters for ease of use, especially in writing. A table of these measures is shown below:
Length  Symbol  Relationship to metre  Typical item of approx size, etc. 
metre  m  1  Height of door handle in the home 
centimetre  cm  1 m = 100 cm  Diameter of shirt button 
millimetre  mm  1 m = 1 000 mm  Thickness of a compact disc; thickness of a UK 5 pence coin 
micrometre (aka micron) 
µm  1 m = 1 000 000 µm  Size of hole in vacuum filter; diameter of human hair can range from 17 to 181 µm 
kilometre  km  1 000 m = 1 km  Central span of the Forth road bridge 
Height is the same as length, but measured vertically. Heights can be measured in centimetres, metres and kilometres. And from length we get speed, which is length per unit time, e.g. kilometres per hour. Click here for speed info page.
Here are some examples of items and their lengths/heights shown in metric units; at least some of these things you have probably encountered, the metric dimensions will help provide a feel for such sizes as 90 mm or 100 metres:
Floppy disk: 90 mm / 9 cm  
Compact disc: 120 mm  
Credit card: 85.5 × 54 × 0.8 mm  
Matchstick: 2 mm wide  
Football pitch (soccer): 100 metres long
Every fullsized soccer pitch must be between 91119 metres long, and between 4691 metres wide. 

Average computer monitor, viewable screen size measured diagonally, on CRT monitor: 41 cm
(Tube size is larger) 

Height of the Eiffel Tower: 300 metres  
Original height of the Great Pyramid of Giza: 147 metres  
Height of the Empire State Building: 448 metres  
Height of average British man: 176 centimetres  
Height of average British woman: 162 centimetres  
Average length of new born baby: 50 centimetres  
London doubledecker routemaster bus: approx 8.4 metres long, 4.4 metres high, 2.4 metres wide.
Modern London buses vary considerably in length, height and design. 
United Kingdom coins
£1 coin Diameter: 23.0323.43 mm 
£2 coin Diameter: 28.4 mm 
50 pence coin Diameter: 27.3 mm 
20 pence coin Diameter: 21.4 mm 
10 pence coin Diameter: 24.5 mm 
5 pence coin Diameter: 18.0 mm 
(coin images and data from the Royal Mint website)
If you are interested in the smaller sizes, have a look at the interactive animation at http://www.cellsalive.com/howbig.htm which will even show you nanometres.
Distance
Distances between cities, for runs, marathons, and longer distances can be measured using kilometres. As we see above, one kilometre is 1000 metres in length.
Some distances in kilometres:
Location from/to  Distance  Notes 
Circumference of the Earth through the poles  40 000 km  This is what the metre was originally based on, but due to slight inaccuracies the exact distance is slightly different (actual length varies according to which website you look at). 
Circumference of the Earth around the Equator  40 075 km  Distance around the Equator is slightly more due to the bulging of the Earth 
Length of Great Britain  1000 km  One tenth of the distance from the North Pole to the Equator 
Length of coastland of Great Britain (mainland only)  17 819 km  
National Grid squares 
100 km by 100 km  Ordnance Survey in the UK produces all the official maps of the country, and divides the country into 91 grid squares, each with two letters, and covering a distance of 100 km on each side of the square. Using this grid and its subdivisions, any place in the UK can be described using its grid reference. More info.
The grid is divided smaller into 10 km and 1 km grid squares. The OS has been using this metric grid since the mid 1940s. 
Astronomical Unit  149 000 000 km  The average distance between the Earth and the Sun, 149 million kilometres. 
Light Year  9 460 000 000 000 km  The distance travelled by light in one year, 9 trillion 460 billion kilometres. 
Minimum height of a mountain in England, Wales, Ireland  610 metres  This is the definition of what constitutes a mountain. If it is smaller than this height, it is not a mountain. Scotland has different classifications. 
Typical altitude of airliner  11 km  
The edge of space  100 km  The Ansari X Prize was won by SpaceShip One when it reached the edge of space twice. 
Distance to the Moon  From 356 371 km to 406 720 km  The Moon’s orbit is elliptical so the distance varies 
Marathon 
42.195 km  The official marathon length, however some running races called marathons may be different distances, such as 40 km, 50 km, and is estimated to have been originally 34.5 km in ancient Greece. 
UK distances  Measured as a straight line, not road distances  Road distance using most likely route 
LondonManchester  263 km  325 km 
LondonEdinburgh  534 km  663 km 
LondonYork  280 km  340 km 
LondonNewcastle  398 km  459 km 
ManchesterEdinburgh  281 km  359 km 
Total length of motorways in Great Britain  4353 km  
Total length of A roads in Great Britain  48 164 km  
Total length of B roads in Great Britain  30 216 km 