Combustion is basically the burning of fuels and oils. When you burn something you need oxygen for the flame(s) to breathe. When there is enough oxygen round when you are burning something it will then be complete combustion.
How do you tell if it is complete combustion?
You can tell if what you are burning underwent complete combustion if the flame is very light. Almost invisible. When the flame looks like that it means that complete combustion is occurring. Complete combustion produces Carbon Dioxide (CO2) and Water(H2O), another way to tell if the combustion is complete or not.
So is there an incomplete combustion?
Yes there is. There are 2 different types of incomplete combustion. Number 1: When there is a small amount of oxygen, the flame will burn an orange/yellow colour. Because there is less oxygen it cannot produce Carbon Dioxide which is 1 carbon atom and 2 oxygen atoms so it will produce Carbon Monoxide. Carbon Monoxide has 1 carbon atom and 1 oxygen atoms. So therefore on of the things that incomplete combustion can produce is Carbon Monoxide and water.
So what is the 2nd thing it can produce?
The second thing it can produce happens when there is a very tiny amount of oxygen. When this happens an orange/yellow flame will burn but there will also be some black smoke. This black smoke is carbon. Also know as soot. Soot and water are produced from incomplete combustion. Soot and Carbon Monoxide are toxic gases and are bad for human respiration.
Why?
Well, carbon monoxide will go into your lungs as you breathe in and will 'cling' to the red blood cells that are supposed to carry oxygen around your body. If the carbon monoxide is there already there is no room for the oxygen and your body won't be able to breathe. Thus suffocating you. Soot does the same as well.