Welding and soldering are two different words with different meanings. However, welding and soldering have to do with joining of metals. Welding is the process of making a joint between metals, this requires a lot of heat in order to make a strong joint. Welding is required when making a structure that must be mechanically fit to carry load and endure stress. In the process, the base metal is melted, welding doesn't necessarily require the use of filler material.
While soldering is the process of making joint between components that are not too strong. The process does not require too much heat, filler materials are used in soldering so as to make a joint. Without filler material there is no soldering. The process is carried out at a temperature below melting temperature unlike welding that uses a temperature higher than the melting temperature. Soldering is mostly used in electrical industries to make joint in electrical components like transistor, electronic panel, capacitor etc