Carbon Monoxide is produced from the incomplete burning of such fuels as: coal, wood, charcoal, oil, kerosene, propane, and natural gas. Carbon Monoxide is produced in many household appliances such as furnaces, stoves, water heaters, and portable headers. Carbon Monoxide poisoning occurs when the devices malfunction or are improperly vented.
The Consumer Product Safety Commission has an excellent information page on what it is, and how to recognize the symptoms of Carbon Monoxide poisoning.