The millimetre (American spelling millimeter) is a unit of length and is a multiple of the SI unit metre. It uses the symbol mm.
There are 1000 mm in 1 m and 10 mm in 1 cm. From this one can derive that there are 0.03937 in.
The millimetre is sometimes verbally abbreviated to 'mil' - which can cause confusion in the US as there is a unit there that means thousandth of an inch.
The micrometre (American spelling micrometer) is a unit of length and is a multiple of the SI unit metre. It uses the symbol μm.
It represents one millionth of a metre and is also known by its previously depreciated name micron.
In the US, the term 'micron' helped distinguish the unit name from the device 'micrometer' but the International System of Units (SI) discontinued this term in favour of micrometre as the prefix system required the use of the Greek symbol 'μ' which invoked the factor x10−6.