There are many types of welding, but four are very common in the modern work environment.
They are:
tungsten inert gas (TIG) welding: typically used for thin, non-ferrous metals such as aluminium, copper or lead.
metal inert gas (MIG) welding: common in the automotive sector, MIG welding involves the heating up of a thin wire as an electrode.
shielded metal arc welding (SMAW): uses an electric current to weld and is commonly used on iron and steel, especially in construction.
flux-cored arc welding (FCAW): a fast form of welding using portable equipment; it's often used in outdoor jobs.