The tonne is a unit of mass (acceptable for use as weight on Earth) and is a non-SI metric unit. It has the symbol t.
1 tonne is equal to 1000 kg. In the US it is called a metric ton (to avoid confusion with other similarly named units like the short ton, the long ton and the register ton. It is equal to approximately 2204.6 lbs.
Tonne is the correct spelling in England and France although ton is acceptable across the world. Before the establishment as an SI-derivative (or metrication), the ton (or imperial ton) was equal to 2240 avoirdupois pounds (or 20 hundredweight). This is only 16 kg different from the up to date (metric) tonne as we know it today.
A car weighs about a tonne.
The milligram is a unit of mass (acceptable for use as weight on Earth) and is a submultiple of an SI base unit with the symbol mg.
1 milligram is equal to 0.001 g or 1/1000 g (one thousandth of a gram).
There exists 2 submultiples between mg and g - namely the decigram and the centigram but these are not commonly used in everyday life. The milligram is commonly used when expressing parts of a gram; especially in medicine applications.
A mosquito weighs about 2.5 mg, as does a grain of sand.