Why are some materials more flammable than others?
Firstly, for a material to be flammable it needs to contain something that is oxidisable by air oxygen, typically carbon. This explain why hydrocarbons and other carbon bearing materials typically burn well but minerals, rock, glass, water etc do not. In the latter materials the elements are already in a high Oxidation State and cannot be further oxidised.

For a generic hydrocarbon the oxidation reaction by air oxygen is given by the following reaction equation:

$$\ce{C_nH_{n+2}(g,s,l) +\frac{3n +1}{2}O_2(g)-&gt; nCO_2(g) +\frac{n +2}{2}H_2O(l)}$$

The second major factor that affects flammability is volatility. The more volatile a material is, the more flammable it will be. The hydrocarbons illustrate that well. Methane is the most volatile and flammable of all of them and by the time we get to the solid ones like paraffin wax, flammability has been greatly reduced because these aren't volatile at all.