The term leading is derived from the days of hot metal type when strips of lead were placed between lines of type to provide line spacing. Leading is the space between lines of type. It is generally measured from baseline to baseline and expressed in points.
Some software may use the term line spacing while others still refer to this spacing as leading. Word processing software often has the option to use single, double, or even triple spacing or to specify specific leading in points or other measurements. Programs that offer automated leading calculate what it believes to be the correct leading based on the text size. When a line of type includes more than one type size, this automatic leading can result in odd or inconsistent line spacing.
Changing the leading of text affects its appearance and readability. Some fonts read better with increased leading due to long ascenders and descenders. Some software has a feature called auto leading which calculates leading automatically.