So I've become interested in sigils recently, and while looking into them I saw that they were a part of chaos magick. What is that and how do sigils fall into that category? I haven't heard the term "chaos magick" before.
Chaos magick is essentially magick without dogma, or magick which is not confined to dogma and beliefs- you can use multile belief systems or create one yourself. There is a lot of rubbish written on the subject, but Phil Hine and Peter Carroll are names which are good to check out.
As for sigils- you create your own sigil utilising your own subconscious, no deity, no belief- in this case anyway.
The best book I have come across on sigil magick is the one written by Frater U.D.