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.
The em is a unit of length in the field of typography and uses the symbol em.
It is a "relative" unit and equals the currently specified point size. For example, 1 em in a 24-point typeface is equal to 24 points. This allows a typographer to specify text size relative to a base figure.
In metal type, the em was equal to the line height of the metal body. This meant the physical size of the letter could not exceed the em. In digital type, the em is a grid of arbitrary resolution so can be scaled to any point size for screen or print.