The link that @praecog29 provided above has some useful methods to work with sigils.
Regarding what they are exactly, here’s my reply another topic where we were discussing deities and sigils:
Historically, noblemen had a “sigil ring” which was a seal (stamp) built into a ring. This sigil was a sign of authority which they could use to stamp (wax) on important documents as proof of authenticity.
The Seal of Solomon, for example, comes from the legend of the signet ring attributed to King Solomon in medieval Arabic tradition. There are many versions of it, here’s one:
The term sigil derives from the Latin sigillum, meaning “seal”.
The word sigil… has a long history in Western magic. The members of the Golden Dawn were perfectly familiar with it (″combining the letters, the colours, the attributions and their Synthesis, thou mayest build up a telesmatic Image of a Force. The Sigil shall then serve thee for the tracing of a Current which shall call into action a certain Elemental Force″) and it was used in the making of talismans. The sigil was like a signature or sign of an occult entity.