What is a micrometer?
A micrometer (µm; micrometre outside the US) is 0.000001 meters. The µ is the Greek letter mu, the SI symbol for the prefix micro.
A micrometer is a millionth of a meter, the scale of things you need a microscope for. A red blood cell is about 7 micrometers across and a human hair about 70.
The unit shares its name with the micrometer screw gauge, the hand tool for measuring small parts. That usually reads to 0.01mm, which is 10 micrometers, so the micrometer mostly doesn't measure in micrometers.
What is an inch?
An inch is a length unit normally used in the United States and, occasionally, in the United Kingdom. It is defined as a twelth (1/12th) of a foot. The inch is normally subdivided into fractions of an inch such as the ½, ¼ and ⅛.
An inch was originally the width of a man's thumb. It is still the everyday length unit in the United States, and TV and computer screens are sold by their size in inches worldwide.
Inches are used in combination with other units of imperial measurement such as feet and yards. For example, a standard door in the United States a football field is 100 yards long or 300 feet or 3,600 inches.