Kerning, leading and tracking are techniques used in typography to enhance the appearance of text primarily to improve readability.
The spacing between two letters or selective letter spacing. There are a few combinations of letters that usually work well when kerned properly to reduce the space between them such as, AT, TI, WO to name a few. Kerning plays an important role when using large type sizes and all caps.
Adjusting the space between letters in a word equally. While kerning is used to adjust the spacing between two letters in a word or the spacing between words, in tracking you adjust the spacing between all the letters in a word equally.
How the text is spaced vertically, that is the space between the lines. It is measured from the baseline of a line to the baseline of the line above it. Usually, leading is 20% greater than the font size, however it can vary depending on the font in use.
Kerning, tracking and leading will depend on the typeface, type style and type size in use and usually require only minimal adjustments. Spacing can affect the colour of the typesetting, a tight spacing will look more black while a looser setting looks more grey. It is a matter of visual adjustment over any mathematical approach.
