CSS measurement unit

CSS has two forms of measurement, absolute and relative. As a general rule, relative units are preferentially used for responsive and mobile design; absolute units tend to be used for fixed-width designs.

Absolute		Relative
CSS Unit	CSS Abbreviation	CSS Unit	CSS Abbreviation
Pixels	px	Percent	%
Points	pt	Em	em
Inches	in	Ex	ex
Centimeters	cm	Root em	rem
Picas	pc	Viewport width	vw
		Viewport height	vh
		Viewport minimum	vm
		character	ch
		Grid	gd

rem, vh, vw are CSS3 measurement units, supported by modern browsers.
1em is the width of the m character in the default font set for that browser. (ex is the height of the x character in a font). rem allows you to set a root font size for size for the entire page, with sizes for elements relative to this measurement:

html { font-size: 62.5%; }

body { font-size: 1.4rem; }

h1 { font-size: 2.4rem; }

(The measurements above are used because they map exactly to pixels: with a base font-size of 62.5% for a page, the body size maps to 14 pixels, and h1 to 24 pixels.)
The viewport is the browser window: vh, vw and vm can scale elements to its current size. This could be used to restrict the upper scaled size of images, for example:

img { max-width: 90vw; }

ch is the equivalent of em, but applied to the width 0 numeral of the chosen font.
gd relates to the CSS3 grid; unfortunately, only IE10 browser yet supports the grid module at this time.
You never need to specify units when declaring a value of 0: zero centimeters is the same as 0 pixels. In all other cases, units of measurement should be declared 2em, 2px, etc, not 2.
With the exception of pixels, all systems of measurement can take floating point values: 2.25em, 5.3cm, etc.
Generally speaking, CSS written for screen display devices should use relative units (percent, em and ex), with few exceptions (such as fixed-with bitmapped images, and hairlines (i.e. 1 to 5 pixel width borders)).