The kilometre is a unit of length and is a multiple of the metre; an SI derived unit. It has the symbol km.
A kilometre is equal to 1000m, 1094 yards and 0.621 miles.
Kilometres is used to measure road distances in the US, though the UK is still using the imperial "sister" miles.
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.