In addition to the ones itsbob mentioned there is also spot welding, resistance seam welding, friction welding (often inadvertently which is when it is called galling.), laser welding, electron beam welding, thermite welding. all have their strengths and weaknesses and often the kind of welding chosen is often a function of cost or access. some welding processes like laser and EB are very sensitive to part geometry and in the case of EB also must be done in a vacuum chamber. Both however offer very deep penetration (several inches in come cases and very small HAZ (Heat Affected Zones). friction stir welding is the new kid on the block. you have to be able to mount the part on a high torque bridge mill and it is not widely used except on alloys like 7075 aluminum which is difficult or impossible to weld any other way. Some like GMAW and GTAW (MIG and TIG) are used on alloy systems that must be welded in an inert atmosphere (argon helium or a mixture) like nickel alloys, Stainless steels, Aluminum (Helium is used and it is also known as heliarc welding) and Titanium. Spot welding is used throughout car assembly as is FCAW (flux core wire welding, very similar to MIG but without the separate shielding gas bottle. the flux creates the shielding gas much the way it does for "stick welding").