The inch is a unit of length in the imperial unit system with the symbol in.
There are 12 inches in a foot and 3 feet in a yard. Therefore there are 36 inches in a yard. There are exactly 2.54 centimetres in an inch.
Borrowed from the Latin 'uncia' - the English word 'inch', the origination of the word came from the Old English word for 'ounce' which was related to the Roman phrase for "one twelfth".
The inch is still a commonly used unit in the UK, USA and Canada - and is also still used in the production of electronic equipment, still very evident in the measuring of monitor and screen sizing.
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.