In writing and typography, a ligature is when two or more graphemes (~letters) are joined as a single glyph. Ligatures usually replace two characters next to each other. The idea comes from handwriting and manuscripts.
Example: the common ampersand ("&") represents the Latin conjunction et, meaning "and". The ampersand's symbol is a ligature, joining the old handwritten Latin letters e and t of the word et, so that the word is represented as a single glyph.
Sometimes ligatures are made to make things more easy to read, or simply to make the writing/font nicer to look at. This is important in some forms of design work.
Type designers, who create fonts (or 'typefaces'), make ligatures for some combinations of letters. Today, most fonts are made on computers (these are called 'digital typefaces'). When two particular letters are typed together, the ligature will appear. The ligature look like the two different letters but slightly changed (sometimes joined together). It is actually a separate symbol that has been drawn by the creator, and is programmed to appear when two letters are put next to each other.
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Ligature (typography) Facts for Kids. Kiddle Encyclopedia.