• Welding:

When you weld two materials to each other, you attach them to each other by melting the attached parts and the welding material into each other. Basically a good welder creates one single piece from the welded parts.

You can only weld two or more identical category of materials to each other. For example, you cannot weld a tungsten-carbide bit to a steel piece.

• Soldering:

It is used to attach two or more pieces of different category of materials to each other. When you solder two pieces to each other, they do not meld and become one. Instead, they are sort of "glued" to each other.