An illustration is a picture that depicts or enhances a piece of text. Illustrations are used for the decoration or explanation of a text, or to reinforce concepts and thoughts delivered in written compositions.

Fictional works use mainly pictorial forms of illustration, to help the reader visualize imaginary characters or scenes described by the narrative, whereas non-fictional works tend to use photographic, statistical and technical diagrams to enhance the information given. Children's books often have many illustrations, as it makes the stories easier to understand. Illustrations within books, are called 'Plates'.

The term 'illuminated' stems from the Latin, 'illuminare' in prosaic context meaning to 'adorn'.

Although the word manuscript brings forth a vision of written parchments, early handwritten and decorated books are also referred to as such.

The decoration of the manuscripts was quite varied; including dramatic border work, initial decoration (scenic initials are referred to as 'historiated initials') and sometimes miniatures or small pictures often not related to the text, but placed simply within it to please the eye. Much of the decoration was moralistic in nature, due to the fact that almost all early works were of a religious nature.

The illumination and written composition of manuscripts was undertaken by monks, and the skills needed were learnt over many years by the individual. A special room, called a 'scriptorium' was designated within a monastery; and it was in rooms such as these, the monks scribed, decorated and gilded such masterpieces as the 'Book of Kells'.

In the late Middle Ages, skilled illuminators were not only found in monasteries, indeed they were everyday citizens; craftsmen usually found within the larger towns or cities, where book production was concentrated.

Manuscripts were generally written on parchment, skin and vellum, the decorative ink colours derived from natural sources such as azurite (blue) and malachite (green).

Some illustration types apply to purely digital use, some to both digital and hard-copy.

Illustration types:

- 'Doodles'
- Line drawing and sketches
- Grey scale and half tone images
- Color graphics
- Technical or statistical diagrams
- Perspective drawings in parallel or with vanishing point views
- Exploded views (isometric and dimetric projection)
- Photographs and enhanced photographs
- Schematics in 2D or 3D
- Comic, strip and other inventive artwork
- Icons, avatars and other small graphics
- Presentation graphic sets
- Animated graphics
- Advertising graphics
- Complete graphic sets for web-based use
- Text enhancement and decoration