Indices : Terminology : Ligature

The connecting link between two letters that are joined together. Also, two or more characters that are combined into one glyph, generally comprising such characters as fi, ff, fl, ffi, ffl etc.

When type was made of metal, ligatures were a physical necessity to stop one piece of metal clashing with another: it was physically impossible for strokes to overlap. Not so with digital strokes: so nowadays, ligatures have primarily aesthetic benefits. Some ligatures, such as the common fi may merely prevent the collision of two glyphs, but others such as the OO ligature provide a decorative, ornamental quality.

Discussion:
qu ligature, tell me why not? descends into a discussion of what comprises a ligature exactly.
Non-Latin Ligatures
Bring out Your Ligatures

Technical Discussion:
Unicode Ligatures

Additional Articles:
Wikipedia > Ligatures