the way I understand it is:
A sigil (pronounced /ˈsɪdʒ.ɪl/ or /ˈsɪg.ɪl/; pl. sigilia or sigils) is a symbol created for a specific magical purpose. A sigil is usually made up of a complex combination of several specific symbols or geometric figures each with a specific meaning or intent.
The term sigil derives from the Latin sigilum meaning "seal," though it may also be related to the Hebrew סגולה (segulah meaning "word, action or item of spiritual effect"). The old Norse binding rune is an example of the idea. A sigil may have an abstract, pictorial or semi-abstract form. It may appear in any medium -- physical, virtual, or mental. Visual symbols are the most popular form, but the use of aural and tactile symbols in magic is not unheard of.
Sigil magic is a common form of magical work among practitioners; symbols and signs have always been a tool of magicians and alchemists. In modern uses, the concept was mostly popularized by Austin Osman Spare, who published a method by which the words of a statement of intent are reduced into an abstract design; the sigil is then charged with the will of the creator. Spare's technique, now known as sigilization, has become a core element of chaos magic. The inherently individualistic nature of chaos magic leads most chaos magicians to prepare and cast sigils in unique ways, as the process of sigilization has never been defined rigorously and the magician is expected to 'fill in the blank spots' by his or her self. Sigils may be used for any purpose or desire, no matter how trivial or important.
More here:
http://en.wikipedia.org/wiki/Sigil_%28magic%29
Blessed Be,
Ostara_Witch)O(