The use of symbols for magical or cultic purposes has been widespread since at least the Neolithic era.
Sigil Magic is a widely used form of magical spellcasting, common to many cultures throughout history. Norse Bind-runes, Arabic charms and the Kabballist's Khem are historic examples of using a written alphabet as a way of devising magical talismans.
Sigils are used for spells as well as for the creation of thoughtforms.
Simply begin at the circle, and follow the line to each letter to spell out your desires. The one in the center is "health" and the one on the bottom is "joy." This can be used during the circle casting part of your ritual to write the names of the four elements at each point of your circle, to write your goals of the spell, etc. It can be used alone, or with the spoken chants and spells that extra power
Symbols have a much stronger impact on the subconscious mind than mere words, and spellcasting done using this method can be done in complete silence, as well as being pretty much unreadable by anyone who doesn't know the chart!
Differences from traditional use
Unlike with traditional sigils, whose creators made use of traditional lore passed down from generations or from books, modern users often create sigils entirely themselves and devise individual means of "charging" them with metaphysical power.
Modern sigils may appear in any medium—physical, virtual, or mental. Visual symbols are the traditional, and presumably still most popular, form, but the use other's are sometimes found in magick.
A modern personal sigil
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 (or "charge") sigils in unique ways, as the process of sigilization has never been rigorously defined. The magician is expected to "fill in the blank spots" by himself or herself.