Now, I’m not going to get into the process of welding and soldering (pronounced SAUTERING), but rather the difference between the two, as they are normally pretty confusing.
Welding is really just the melting, or fusing, of two or more metals together. In industrial welding, the torch applies extreme heat to only a specific area, and a rod of the same metal that is being welded is held against the seam. The torch slightly melts the two edges, as well as the rod being applied there. They all melt together (the rod fills in the cracks), and when they cool, become one big piece of metal. One more note about welding, is that its definition is melting two metals together for crafting purposes. Welding includes hammering red-hot pieces of metal together, just as long as they are melted together, then it is welding.
Soldering is when the two metals to be joined are heated up, though not to melting, and a third metal, the solder is melted on. The same amount of heat is applied to all three metals, but the solder melts at a lower temperature than the other two. The molten solder flows and the smallest layer of molecules forms an alloy with the metals to be joined, thus filling in the cracks between them, and grabbing on to them to hold the two metals together as well. The solder can be melted off very easily, without damage or defect to either of the two joined metals.
So you see, there are benefits of both. Welding is much stronger, and is tended to be used only on steel and iron for big industrial uses. Of course, the melted metal gets all blobby-looking after the welding is done, and does not look very clean or beautiful. Soldering is used in jewelry, or in piping, where precision is essential. Soldering is very clean, beautiful, and easy to clean off, but it is not very strong, as the solder itself is a very weak alloy.