Three things have to combine to have a successful campfire. Oxygen (in the air), Fuel (most often wood) and heat (source of ignition). This is called the "Fire Triangle". Knowing this concept helps both with starting campfires and putting out the fires you don't want.
Below is a simple diagram of the Fire Triangle and then some questions about the Fire Triangle.
Wikipedia has a more detailed explanation if you need more information: https://en.wikipedia.org/wiki/Fire_triangle