Using correct polarity makes the difference between a quality weld, and one that is porous and weak, or one that gets proper penetration, or one that barely gets any at all. Beyond the confusion of the terms, it is often confusing to know just when to use what polarity.
While stick (SMAW) welding, the great majority of welding rods require a positive polarity. In fact nearly all welding rods run primarily with positive, (reverse) polarity. With a few exceptions, in the case of running on thin sheet metal, you should always run with reverse polarity. Since the term “Reverse” may be confusing, just always think “positive torch” and you should be ok. MIG welding is always going to be positive polarity. Some people confuse MIG with flux core welding. They are not the same, though the equipment used can be nearly the same or identical. Flux core uses a wire that contains a powder flux in the center of the wire. It has its advantages in windy conditions, and offers go anywhere performance. While not necessarily better or worse than the MIG process, it does utilize straight polarity, just the opposite of MIG. There may be some wire manufacturers however that recommend positive polarity, so be sure to check the label. One thing to note, MIG and Flux Core wire do not have “optional” polarity with the same wire. If its positive, it will only run successfully on positive. Dual shield wire utilizes the best of MIG and Flux Core by using a flux wire and shielding gas. Always follow the polarity recommendation of the wire manufacturer of course, but generally dual, or outer shield wire uses reverse polarity.
TIG is the exception and is the process where it will always weld with the torch in the negative terminal (even if welding in AC mode there are reasons to keep the torch plugged into the negative side). Years ago, DC positive was used for welding aluminum, but penetration was very poor, and it required enormous diameter tungsten to weld at relatively low amps due to heat being maximized on the tungsten tip instead of the base metal. Since aluminum welding is so easily accomplished with AC tig units with either high frequency overlay or inverter style welders such as the Everlast PowerTIG series, DC negative torch polarity is the only one used.
Whatever the polarity, you may need to employ some sort of memory device to help you remember the polarity terms. Even if you have to write a note on your welder to help you remember the “torch” side that you will use, it will help you in get it right, and keep frustration down to a minimum.