Welding is a fabrication process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material (the weld puddle) that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. For an overview of the welding technology development, have a look on the following links.
URL 1 is the overall picture of welding and it's history
URL 2 is somewhere you can ask questions regarding MMA, MIG, TIG welding tips.
URL 3 is welding related supplies shop that I think is really good for DIYers.