Surface Mounted Technology
The legs of components that are made using 'Surface Mounted Technology' are soldered to the conductor pattern on the same side of the PCB as the component is mounted. This technology does therefore not require a hole in the PCB for every leg of the component.
Surface Mounted Components could even be mounted on both sides of the PCB directly underneath each other.
SMT components are also much smaller than THT components. This makes PCBs with SMT components much more dense compared to similar PCBs with THT components. Today SMT components are also cheaper than THT components. It is therefore no surprise that most components on main boards nowadays are SMT.
Since the connection points and component legs are so small it becomes very hard to solder on a SMT component manually. Considering that machines do almost all assembly, this issue only becomes important when repairs have to be done.
The design of a PCB is a process that starts long before the actual routing of the conductor pattern. These are the main steps in the design: