What Is Ligature?
A ligature is a single glyph that combines two or more letters, designed so awkward collisions disappear. The classic example is "fi," where the dot of the "i" clashes with the hook of the "f." Standard ligatures improve readability; decorative ones add flourish to scripts and display type.
Standard ligatures (fi, fl, ff, ffi) fix mechanical collisions and are usually on by default in good software. Discretionary ligatures (like ct or st with a connecting swash) are decorative and opt-in.
In OpenType, ligatures are extra glyphs swapped in automatically as you type the underlying letters, so the text stays editable and searchable.