one type of bridge is a beam bridge.  beam bridges have several beams supporting them.  some of the loads that a beam bridge would hold would be people, trains, cars, and any moving thing.  the beam bridge has compression and tension. 

a truss bridge has one or two beams and trusses supporting it.  some of the loads that a truss bridge has is a train, people, and anything moving or animals.  

an arch bridge is a bridge that is supported by an arch.  the bridges loads are anything that moves, so for example a train or a car.