Introduction to GEANT4

* <span style="color:#800000"> G4VUserDetectorConstruction </span>: Describe the experimental setup, geometry and materials

* <span style="color:#800000"> G4VUserPhysicsList </span>: Define particles, physics processes and range cuts

* <span style="color:#800000"> G4VUserPrimaryGeneratorAction </span>: Describe particle source, source dimensions, initial position, energy spectrum, angular distributions

* <span style="color:#800000"> G4UserRunAction </span>: Define and store histograms  

* <span style="color:#800000"> G4UserEventAction </span>: Event selection and analysis of simulation data

* <span style="color:#800000"> G4UserStackingAction </span>: Customize priority of tracks

* <span style="color:#800000"> G4UserTrackingAction </span>: Decide whether a trajectory should be stored or not

* <span style="color:#800000"> G4UserSteppingAction </span>: Kill, suspend, postpone a track

* <span style="color:#800000"> G4RunManager </span>: Manages the simulation process