What is a throw away (also know as hidden city) ticket?  

Before answering that, travelers should know that non-stop flights usually cost significantly more than connecting flights even though connecting flights may connect through the non-stop cities.

For example, let's say you want to do a R/T flight between Dallas DFW and LIM (Lima, Peru).  A R/T ticket from DFW to LIM departing DFW on April 10 and returning from LIM on April 20 is $1065.  It is a non-stop flight.   Book a flight from Austin AUS airport to LIM with departure and return on the same dates, the fare falls to $821.  However, on the return flight, it stops in DFW before going on to AUS.

A traveler might choose to purchase the R/T flight between AUS and LIM at a savings of $244 per person.  A car rental from DFW to AUS is about $100.  On the return from LIM to AUS the travelers "throw away" the ticket from DFW to AUS and get off in DFW near their home.   The saving compounds itself when there are multiple travelers.  2 travelers save a total of $488, 3 save $732, and 4 save $976.  That justifies the time and expense of driving to Austin.

However, there are risks.  Airlines are on to this traveler strategy and usually prohibit it in their "terms and conditions".  To retaliate, they may try to charge the traveler the difference between the non-stop flight and the connecting flight.  In the case above, they may try to charge the travelers the additional $244 fro throwing away the last segment of the trip.  Some airlines may suspend, cancel, or restrict a traveler's frequent flyer program. In the case of a recent Lufthansa flight, they actually sued a passenger for a throw away ticketing incident.   However, if the passenger only does this on a rare circumstance, there might be little chance of any penalty.