What are Ligatures?

Ligatures are typographic replacement characters for certain letter pairs. Most fonts include ligatures for standard letter pairs such as fi, fl, ff, ffi, and ffl.


Image Source:
https://helpx.adobe.com/illustrator/using/arabic-hebrew.html




Image Source:
https://www.typenetwork.com/news/article/opentype-at-work-contextual-alternates

"Ligatures and contextual alternates basically serve the same purpose: they offer solutions for problematic or unwieldy glyph sequences. Although their name implies that they connect two or more glyphs, some ligatures actually solve problems by shortening letter parts, exactly like some contextual alternates do. So why are they two distinct features? The fundamental difference between them is that ligatures replace two or more glyphs with one combined glyph, while contextual alternates only change the appearance of one glyph at a time. A ligature typically resolves one single awkward glyph combination; a contextual alternate can offer solutions for many different graceless scenarios."


*contextual alternates only change two or more glyphs at a time.

*ligatures replace two or more glyphs with one combined glyph


What is a glyph?

A Glyph is a stylic set that can be applied to a group of characters.

SOURCE:
https://helpx.adobe.com/illustrator/using/special-characters.html

Ligatures - Text

Ligatures are typographic replacement characters for certain letter pairs.

Contextual alternates are alternate characters included in some script typefaces to provide better joining behavior.

A stylistic set is a group of glyph alternates that can be applied to a selected block of text.

Window > Type > OpenType to open the OpenType panel. Now, do one of the following:
Click the Stylistic Sets icon () at the bottom of the panel and choose the desired set.
Select Stylistic Sets from the panel menu and choose the desired set.

SOURCE:
https://helpx.adobe.com/illustrator/using/special-characters.html#use_ligatures_and_contextual_alternates