The best way to get a fast overview of the HepMC event record is to browse the first 3 pages of the user manual (linked here). The HepMC package is an object oriented event record written in C++ for High Energy Physics Monte Carlo Generators. Many extensions from HEPEVT, the Fortran HEP standard, are supported: the number of entries is unlimited, spin density matrices can be stored with each vertex, flow patterns (such as color) can be stored and traced, integers representing random number generator states can be stored, and an arbitrary number of event weights can be included. Particles and vertices are kept separate in a graph structure, physically similar to a physics event. The added information supports the modularisation of event generators. The package has been kept as simple as possible with minimal internal/external dependencies. Event information is accessed by means of iterators supplied with the package.
M. Dobbs and J.B. Hansen, Comput. Phys. Commun. 134 (2001) 41.