Combustion (or burning, as it is more commonly known) is the reaction between a fuel and Oxygen which can create lots of energy.
The Oxygen required for this reaction comes from the air. The air is a mixture of gases. It is mainly composed of: Nitrogen (about 78%), Oxygen (21%) and a small mixture of other gases such as Carbon Dioxide and Argon.
During combustion, oxides are also produced.
For example, when the flammable metal magnesium is burned, lots of light, heat and smoke are produced, but when the reaction is complete the white solid Magnesium Oxide will be clearly seen. This reaction can be seen in the 'sparklers' that are used on Guy Fawkes night.
This reaction can be shown in a word equation:
Magnesium + Oxygen Magnesium oxide
Please note that the light, heat and smoke are not included in the chemical (word) equation because they are not chemicals and all combustion reactions are irreversible.
The energy produced by combustion can be put to many uses - from heating your home and cooking your food, to powering your car. The fuels used to provide energy for these uses are not metals, like Magnesium but they are more likely to be petrol, diesel, natural gas etc...