Metal fabrication is an umbrella term for any process that transforms raw metal materials into a finished product. It consists of many different techniques, including welding. Aside from welding, fabrication can also consist of bending, stamping, punching, and many more.
Welding is the process in which two pieces of metal are joined together. Welding uses a power supply to create an electric arc between the welding rod and the material itself. This current produces a large amount of heat which will melt the metals. Afterwards, a filler is added to fuse the metal to create a joined piece.