What exactly is "loan fraud," or "mortgage fraud"?

A few examples, per the FBI are:

• Property Flipping - purchasing a property, falsely appraised at a higher value, and then quickly selling it.
   
  
• Silent Second - buyer borrows the down payment from the seller through a non-disclosed second mortgage.
   
  
• Nominee Loans / Straw Buyers - the identity of the borrower is concealed through the use of a nominee who allows the borrower to use the nominee's name and credit history.
   
  
• Inflated Appraisals - an appraiser works together with a borrower to provide a misleading appraisal.

The full list is here.

The long and the short of it --- don't participate in loan fraud --- it is a criminal act.