The set of all distinct ones with the same incoming and outgoing paths corresponds to the perturbation expansion of a matrix element of the scattering matrix in field theory.  
Each one corresponds to the probability amplitude for the process within it.  The set of all distinct ones with the same incoming and outgoing paths corresponds to the perturbation expansion of a matrix element of the scattering matrix in field theory.  They were first designed to give a simpler understanding of Moller scattering and follow two basic rules.  Space-time trajectories of noninteracting electrons are represented by straight lines, and wavy lines are used to show photons.  FTP, identify these pictorial representations of elementary particles and their interactions, named for the American physicist who devised them.
Each one corresponds to the probability amplitude for the process within it.  
Feynman diagrams