Soldering is the act of joining to metal surfaces with solder. It is most often used in electronics to create an electrically conductive joint, but is also used in plumbing to join pipes.
When making solder joints for electronics, typically between a component and PCB, the goal is to create a small and solid join. Typically an 18-25W soldering iron is used by the hobbyist or in an assembly workshop. Mass produced electronics using flow soldering machines.