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A computer font stores data used to display text: glyph descriptions, spacing measurements, and mappings from text characters to glyphs. An OpenType computer font organizes these into separate tables. Its cmap table maps encoded text characters to default glyph indices; other tables can specify subsequent substitutions and positioning.

The separation permits several representations of a glyph. An outline font stores scalable boundaries, whereas a computer font can also contain images already sampled into pixels. The rendering system uses the selected representation and placement data to produce the visible text. A file containing glyph outlines therefore supplies more than a list of Unicode numbers.^[ref]

A font supplies a coordinated collection of glyphs for displaying text. A typeface describes the visual design; a computer font is a digital resource implementing a selection of that design and the data needed to use it. The stored data can include glyph shapes, spacing measurements and rules for substituting or positioning glyphs.^[ref]

OpenType is a table-based computer font format separating glyph representations, character mapping, measurements, naming metadata and optional layout rules. Its cmap table selects default glyph indices for encoded text characters; substitution and positioning tables support more elaborate text shaping.

An OpenType computer font can contain TrueType outlines or Compact Font Format outlines, and the specification also provides for bitmap and other glyph representations. The file format therefore does not prescribe a single kind of outline geometry. Extensions such as .ttf and .otf are useful conventions, but the actual table contents determine the available representation.^[ref]

This organization lets one system answer separate questions: which glyph represents this text, how much space should it occupy, and how should it be drawn at this size? A language's contextual forms can change the first answer without changing the original Unicode text.