A surface-mount device or SMD is an electronic device for which the components are mounted or placed directly onto the surface of the PCB.
Surface-mount technology, or SMT, is the name of the method used for producing an SMD. In the industry it has largely replaced the through-hole technology construction method of fitting components with wire leads into holes in the circuit board.
Both technologies can be used on the same board for components not suited to surface mounting such as large transformers and heat-sinked power semiconductors.
An SMT component is usually smaller than its through-hole counterpart because it has either smaller leads or no leads at all. It may have short pins or leads of various styles, flat contacts, a matrix of solder balls (BGAs), or terminations on the body of the component.