A chemical component used in shower gel, shampoo, and other similar products. It is apparently also used in industrial applications for de-greasing (eg engines).
It is known to cause skin irritation if in contact with the scalp for too long, and can also cause nausea if ingested.
Links between SLS and cancer are unproven, according to Research by Health Canada (http://www.hc-sc.gc.ca/ehp/ehd/psb/cosmetics/sls.htm)