A beam is a horizontal, or nearly horizontal, structural member that is capable of withstanding load primarily by resisting bending. It can also simply be an architectural element which is non-load bearing.
A T or L beam is a beam that forms part of a slab construction and acts together with the slab which its carries. Such beams are often T-shaped but may have other shapes, e.g. L-Shape or Inverted-T-Shape.