What is a decimeter?
A decimeter (dm) is one tenth of a meter, or 10 centimeters. The 'deci' is the SI prefix for 10^-1.
The decimeter is rarely used by itself, since people go from centimeters straight to meters. You mostly meet it as the cubic decimeter, which is exactly one liter.
A decimeter is about 3.937 inches, so just under four. Divide inches by 3.937 – or multiply by 0.254 – to go the other way.
What is a micron?
A micron is one millionth of a meter, or 0.000001 meters. Its symbol is µm, the Greek letter mu followed by m.
'Micron' is the older name for the micrometer, and the SI dropped it in 1967. It survives on filter ratings anyway – a 5 micron filter catches particles down to about 5 micrometers.