An em space is a term derived from the width of the capital "M" on a metal character block used with early print presses, similar to the blocks shown in the picture on this page.
Today, an em space is used to define the current point size of a font. For example, a 12-point type has a em space of 12 points, which also means each space is 12 points.
Web designers or anyone working with CSS are usually introduced to the em measurement when defining the size of fonts. By using em values to define your fonts relative size on the web page it is possible to increase and decrease the size of the font relative to the default or sites relative font size.
For example, in the below examples of different em sizes, the 1em is the same size as the text on this page. Increasing the em to a 2em effectively doubles the size of the font and 0.5 reduces the size of the font in half.
This is an example of 2em
This is an example of 1em
This is an example of 0.5 em
The em font size can be used when defining the font size of a web page styel element or within the CSS code. Below is an example of how this could be done in an HTML paragraph tag.
<p style="font-size:2em">This is an example of 2em</p>