I am a Materials Joining Engineering Technology major. (AKA welding)
-Properly made welds can be stronger than the surrounding metal.
-There are almost as many different types of welding as there are needs for welding.
There are three kinds of welding you're likely to see in a basic welding class:
Stick (SMAW- Shielded Metal Arc Welding) - The second kind of welding ever. Essentially, a steel rod covered in a ceramic-like material called flux is connected to the positive connection, the workpiece to ground. An arc is established between the electrode(steel rod) and the workpiece. This creates intense heat that melts both the steel rod, the workpiece, and the flux. The flux creates a gas pocket as it melts, and also slag. The gas protects the molten weld metal from the atmosphere and stabilizes the arc. The slag creates a hard ceramic coating on the weld which protects it as it cools. This is later chipped off, leaving a clean steel weld 'bead'.
MIG - (GMAW - Gas Metal Arc Welding) This is completely different from stick. In this process, a bottled shielding gas and a spool of wire is used. Both the gas and the wire are fed through a hose to the welding gun. The gas shields the weld and stabilizes the arc, just like in stick. The wire carries the electrical current. With low current, the wire will short circuit to the metal and melt in. Higher currents cause a constant arc, and the end of the wire melts off in fairly large drops, and into the weld pool. In even higher currents, tiny droplets form on the end of the wire at a high rate, and are propelled into the weld by the electromagnetism created by the current. There is no slag in GMAW, but a similar type of welding exists called FCAW(Flux-cored arc welding), wherein a flux powder is carried in the core of the wire. This does the same thing as flux in arc welding, and does form slag.
TIG (GTAW - Gas Tungsten Arc Welding) In GTAW, the arc and filler metal come from different places. The welder holds the electrode, which is made of Tungsten. Tungsten can withstand extremely high heat without melting, and also is a very good conductor at high temperatures(thermionic). The arc is initiated between the workpiece and the tungsten electrode, and a weld pool of molten metal is established. Then, the welder feeds in a welding rod, which is just a plain rod of metal of similar composition to the base metal- it may contain alloying elements to make a stronger weld, or just be plain metal like the base metal. GTAW also uses shielding gas from a bottle. GTAW may be DC electrode positive or negative, or also AC.
There are about a bazillion more. If you want to know more about any of them, feel free to ask.