What is a short sale?  A short sale is when a homeowner owes more on their home than their house is worth and they are selling it for less than they owe.  Their lender must approve the short sale and agree to take less. It can be a lenghthy process but if you're patient you may be able to buy a great home at fair market value.  In this case you are dealing with a homeowner and their lender as the seller.

What is a REO?  Short for Real Estate Owned, also know as Bank Owned or Lender Owned.  This is a property that has been foreclosed on and the bank has taken it back into their possession.  In this case you are dealing with the bank as the seller.

What is a standard sale?  This is when a homeowner is not upside down in their home and they have enough equity to pay off their mortgage in full and all costs involved.  In this case you are dealing with the homeowner as the seller.