A typical radical chain reaction consists of three types of steps:

1.  One initiation step
An initiation step is a reaction in which radicals are generated from a stable, non-radical precursor.  Initiation increases the concentration of radicals in the system.

2. Two or more propagation steps
A propagation step is a reaction in which a radical is consumed and a new radical is generated.  Propagation does not affect the concentration of radicals in the system.

3.  One or more termination steps
A termination step is a reaction in which radicals are consumed but no new radicals are generated.  Termination decreases the concentration of radicals in the system.

eg:  Heat-induced reaction of excess methane with bromine