A cantilever is a rigid structural element that extends horizontally and is unsupported at one end. Typically it extends from a flat vertical surface such as a wall, to which it must be firmly attached. Like other structural elements, a cantilever can be formed as a beam, plate, truss, or slab.
The Forth Bridge, a cantilever truss bridge
This concrete bridge temporarily functions as a set of two balanced cantilevers during construction – with further cantilevers jutting out to support formwork.
Howrah Bridge in India, a cantilever bridge
A cantilevered balcony of the Fallingwater house, by Frank Lloyd Wright
A beam is a structural element that primarily resists loads applied laterally across the beam's axis. Its mode of deflection is primarily by bending, as loads produce reaction forces at the beam's support points and internal bending moments, shear, stresses, strains, and deflections. Beams are characterized by their manner of support, profile, equilibrium conditions, length, and material.
A beam of PSL lumber installed to replace a load-bearing wall
An I shaped beam of metal under a bridge