A sigil in the simplest of words is a magical symbol. They are normally created by each energy worker independently so that it can serve a very specific function. Since sigils are typically very specific, they are very rarely re-used; in fact, this is much frowned upon. Sigils, in a way, are man's way of 'bending the rules' in a way that does not require mass bloodshed. Sigils are made with a certain desire; it is built into it. When the sigil is used, that desire comes to fruition.