There are mainly 3 commonly used types of welding that are used today. MIG, TIG, and Stick. All 3 have their advantages and disadvantages depending on the situation and external factors such as weather etc.
MIG WELDING (Metal Inert Gas)
One type is known as Gas Metal Arc Welding, or Metal Active Gas. This is a process in which an electric arc is formed between a wire electrode (feed wire) and your work base metal. This heats , then melts the work metal with the wire (electrode) and joins and bonds them. Along with the wire electrode , there is also a shielding gas which is fed through the gun, which 'shields' the welds from air contaniments. MIG welding can be semi-automatic or automatic. Typically a constant power source is used, but not always required.
MIG or GMAW is the most commonly used process for industrial applications. It is fast , economical and versatile. Alot of robots (auto-welding machines) are applied using MIG because of the ease of the process. Most auto-welding robots are used in the pipe seam process.
TIG WELDING (Tungsten Inert Gas)
Another type of welding is TIG (Tungsten Inert Gas) , or GTAW (gas tungsten arc welding) is a process in which a NON-consumable tungsten electrode is used , (unlike MIG) . This process is usually used on stainless steel, aluminium and other non-ferrous metals. (magnesium and copper alloys). The process is very difficult to master but offers more control and accuracy then other types of welding such as MIG and Stick. However, because of the complexity it is often a much slower process than MIG or stick.
some examples of applications of TIG would be : stainless steel handrail, ornamental rails etc.
Shielded Metal Arc Welding , or SMAW is basically manual arc welding. Or "stick welding". This is a manual process that uses a consumable electrode (the rod) which is coated in flux , to create the weld. An elecrtic current has to be present between the welding power generator and the workpiece, this creates an 'arc' between the electrode and the base metal, fusing them together. As the weld is put down, the flux coating on the electrode rod disintegrates, giving off a vapor that will act as a shielding gas and provides a layer of (somewhat undesirable) slag. This 'slag' protects the weld from the envirmonental contamination.
This is a very versatile method of welding, and is still commonly used even though there are much more 'sophisticated' processess. Stick welding is ideal for outdoor conditions, known as 'field welding' due to its durability in weather conditions.