Force Carriers
In particle processes the forces are described as due to the exchange 
of particles,
called "force carriers"; for each type of force there is an associated 
carrier particle.  They belong to the class of particles, called 
bosons.  
Picture two players tossing a (heavy) basketball back and forth.  
If you were far
enough away,
 you would not see the basketball but you would realize
 that there is something
 that accounts for the players' motion.
The basketball is similar to the carrier particle 
exchanged between two particles.  The force
 carrier  can be produced or absorbed only by particles that have 
"charges" for that interaction.
Force Carriers Chart
Interactions Chart