Methane chlorination is a chain reaction. If only the products and reactants are analyzed, the result is:
However, this reaction has 3 intermediate reactants which are formed during a sequence of 4 irreversible second order reactions until we arrive at the final product. This is why it is called a chain reaction. Following only the carbon containing species in series:
Reactants: 
Products: 
The other species are reaction intermediates: 
These are the set of irreversible second-order reactions:
These intermediate species' concentrations can be calculated by integrating the system of kinetic equations. The full reaction is a free radical propagation reaction which is filled out in detail below.
Initiation: This reaction can occur by thermolysis (heating) or photolysis (absorption of light) leading to the breakage of a molecular chlorine bond.
When the bond is broken it produces two highly reactive chlorine atoms.
Propagation: This stage has two distinct reaction classes. The first is the stripping of a hydrogen from the carbon species by the chlorine radicals. This occurs because chlorine atoms alone are unstable, and these chlorine atoms react with one the carbon species' hydrogens. The result is the formation of hydrochloric acid and a new radical methyl group.
These new radical carbon containing species now react with a second  molecule. This regenerates the chlorine radical and the cycle continues. This reaction occurs because while the radical methyl species are more stable than the radical chlorines, the overall stability of the newly formed chloromethane species more than makes up the energy difference.
During the propagation of the reaction, there are several highly reactive species that will be removed and stabilized at the termination step.
Termination: This kind of reaction takes place when the radical species interact directly. The products of the termination reactions are typically very low yield in comparison to the main products or intermediates as the highly reactive radical species are in relatively low concentration in relation to the rest of the mixture. This kind of reaction produces stable side products, reactants, or intermediates and slows the propagation reaction by lowering the number of radicals available to propagate the chain reaction.
There are many different termination combinations, some examples are:
Union of methyl radicals from a C-C bond leading to ethane (a side product).
Union of one methyl radical to a Cl radical forming chloromethane (another reaction forming an intermediate).
Union of two Cl radicals to reform chlorine gas (a reaction reforming a reactant).