A beam is a structural element that primarily resists loads applied laterally to the beam’s axis. Its mode of deflection is primarily by bending. The loads applied to the beam result in reaction forces at the beam’s support points. The total effect of all the forces acting on the beam is to produce shear forces and bending moments within the beam, that in turn induce internal stresses, strains and deflections of the beam. Beams are characterized by their manner of support, profile (shape of cross-section), length, and their material.
When beams support slabs, they work together forming a “T” section beam. The level of the slabs compared to the level of the beams results in the formation of rectangular beams, inverted beams or “Z” beams. 
Beams are supported by columns (beam to column connection), but ocassionaly one or both beam ends are supported by another beam (beam to beam connection) and other times only one end is supported by a column or beam while the other end has no support at all. Cantilever beam is a projecting beam fixed only at one end.
The beam to column connection is called direct support while the beam to beam connection is called indirect support.
The most usually used beam is the one supported by two columns and the most rarely used is the cantilever beam. Indirect supports should be ocasionly used and only if it’s the only avaible solution. 
