TeX and LaTeX know about these units both inside and outside of math mode.
pt ¶Point, 1/72.27 inch. The (approximate) conversion to metric units is 1point = .35146mm = .035146cm.
pc ¶Pica, 12 pt
in ¶Inch, 72.27 pt
bp ¶Big point, 1/72 inch. This length is the definition of a point in PostScript and many desktop publishing systems.
mm ¶Millimeter, 2.845pt
cm ¶Centimeter, 10mm
dd ¶Didot point, 1.07 pt
cc ¶Cicero, 12 dd
sp ¶Scaled point, 1/65536 pt
Three other units are defined according to the current font, rather than being an absolute dimension.
ex ¶The x-height of the current font ex, traditionally the height of the lowercase letter x, is often used for vertical lengths.
em ¶Similarly em, traditionally the width of the capital
letter M, is often used for horizontal lengths. This is also often
the size of the current font, e.g., a nominal 10pt font will
have 1em = 10pt. LaTeX has several commands to produce
horizontal spaces based on the em (see \enspace & \quad & \qquad).
mu ¶Finally, in math mode, many definitions are expressed in terms of the math unit mu, defined by 1em = 18mu, where the em is taken from the current math symbols family. See Spacing in math mode.
Using these units can help make a definition work better across font
changes. For example, a definition of the vertical space between list
items given as \setlength{\itemsep}{1ex plus 0.05ex minus
0.01ex} is more likely to still be reasonable if the font is changed
than a definition given in points.