The goal of illumination and decoration is to enrich the page of a manuscript.
Illumination may be defined as the decorating by hand, in gold or bright colors, of writing. There are three type of illumination: color-work, pen work (filigree) and limning (painting).
Color-work illumination is mainly a color treatment of a letter or simple abstract form or symbol. Gilding involves laying and burnishing thin leaves of gold on the design. Rubrication is the use of adding red (or other color) to letters in a manuscript. Often it was the initial letter that was illuminated.
Filigree consists mostly of pen flourishes and strokes. Often filigree appears as a vine growing out of the text. First the scribe will "root" the vine in the initial letter and draw branches growing from it. Stems, tendrils, leaves and flowers grow from these branches with the use of a very free flowing pen, into a intricate design.
Limning are miniature paintings. These can be quite elaborate and depend upon the skill of the scribe or artist.
Aside from illuminating the initial letter(s), a manuscript can be decorated with line-finishings to justify the text and/or with a border.
The absence of capital letters in Hebrew provides a unique character of Illuminated Hebrew manuscripts. Instead of an initial letter, initial words are emphasized. The decoration around the word may emphasize the meaning of the word, illustrate something from the text of the page or betotallydecorative and having nothing to do with the printed word at all.