NOD (Notice of Default) Properties
The homeowner has missed enough payments to warrant a formal Notice of Default from the lender.  They have a period of time to bring the loan current before the bank repossess and sells the property.

 NTS (Notice of Trustee Sale) Properties
These are properties that have not been brought current on payment during the redemption period.  The lender hopes to sell these property at the Trustee Sale (public auction) to cover the loan.

 REO (Real Estate Owned) Properties
These are properties that did not sell at The Trustee Sale and they are owned by the bank.