I am buying this house, and the seller has added a 220V outlet to the
garage already in addition to the dryer. Which is nice because I
plan to run a 220V welder or compressor soon..

I did notice he had a honda power generator with a short piece of 220V
cord sitting near the outlet.. which I am assume it plugs into the
outlet.. but I can't figure out what he was doing with it, and I
forgot to ask. Could he have powered the house via the generator
during black outs?

But my question is, will I be able to use this outlet for a 220V
welder?
Does a welder require a different plug than the dryer?