Here's a breakdown:

* Micrometers (µm): This is the standard unit for measuring paper thickness in most parts of the world. One micrometer is equal to one millionth of a meter.

* Mils (thousandths of an inch): This unit is commonly used in the United States. One mil is equal to one thousandth of an inch.

There are also other, less common units used for measuring paper thickness, such as points (1 point = 0.353 mm) and grams per square meter (gsm). However, micrometers and mils are the most widely used.