EN: Features (opentype)
When creating fonts for complex scripts (and even for less complex scripts) various transformations (like ligatures) must be applied to the input glyphs before they are ready for display. These transformations are identified as font features and are tagged with (in OpenType) a 4 letter tag or (in Apple) a 2 number identfier. The meanings of these features are predefined by MicroSoft and Apple. FontForge allows you to tag each lookup with one or several features when you create it (or later).
Font Forge Glossary