Standard sales are owned by individuals. Standard sales can also be owned by corporations that flip properties, the transaction would be the same as one owned by an individual.
REO's are bank owned. Short Sales are owned by the individual also, however, they are close to a possible foreclosure, you first deal with the homeowner who accepts the offer then basically the bank takes over from there, with some minimal participation from the homeowner.
I hope this helped clear things up. Should you need any assistance in locating a property, feel free to contact me.