CSS Units

Absolute Lengths

The absolute length units are fixed and a length expressed in any of these will appear as exactly that size.
Absolute length units are not recommended for use on screen, because screen sizes vary so much. However, they can be used if the output medium is known, such as for print layout.

Unit Description
cm centimeters
mm millimeters
in inches (1in = 96px = 2.54cm)
px pixels (1px = 1/96th of 1in)
pt points (1pt = 1/72 of 1in)
pc picas (1pc = 12 pt)

Relative Lengths

Relative length units specify a length relative to another length property. Relative length units scales better between different rendering mediums.

Unit Description
em Relative to the font-size of the element (2em means 2 times the size of the current font)
ex Relative to the x-height of the current font (rarely used)
ch Relative to width of the "0" (zero)
rem Relative to font-size of the root element
vw Relative to 1% of the width of the viewport
vh Relative to 1% of the height of the viewport
vmin Relative to 1% of viewport's smaller dimension
vmax Relative to 1% of viewport's larger dimension
% Relative to the parent element

Reference:
https://www.w3schools.com/cssref/css_units.asp