The yard is a unit of length in the imperial and US system and uses the symbol yd.
A yard is equal to 3 ft or 36 inches. There is 0.9144 m in a yard. There are 1760 yards in a mile.
Derived from the Old English 'gyrd' or 'gerd', the yard was first defined in the late 1600s laws of Ine of Wessex where a "yard of land" (yardland) was an old unit of tax assessment by the government.
The yard was the original standard adpoted by early English leaders and was apparently used in length by the Saxon race and represented the breadth of the chest of a man. After a relative hiatus, Queen Elizabeth reintroduced the yard as the English standard of measure, and it still survives in many 2nd generation conversations today.
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.