An en is a typographic unit, half of the width of an em. By definition, it is equivalent to half of the height of the font (e.g. in 16 point type it is 8 points). As its name suggests, it is also traditionally the width of an uppercase letter "N".
The en dash (–) and en space ( ) are each one en wide. In English, the en dash is commonly used for inclusive ranges (e.g., "pages 12–17" or "August 7, 1988 – November 26, 2005"), and increasingly to replace the long dash. (Note that when using it to replace a long dash, spaces are needed either side of it – like so.)