Metric kerning uses the kerning tables that are built into the typeface. When you select metric kerning in your page layout program, you are using the spacing that was intended by the type designer. Metric kerning usually looks good, especially at small sizes. Cheap novelty fonts often have little or no built-in kerning and will need to be optically kerned.
Optical kerning is executed automatically by the page layout program. Rather than using the pairs addressed in the font's kerning table, optical kerning assesses the shapes of all characters and adjusts the spacing wherever needed. Some graphic designers apply optical kerning to headlines and metric kerning to text. You can make this process efficient and consistent by setting kerning as part of your character styles.

