In addition to what the previous posted answers have said, when you are behind in payments you will most likely be inundated with letters, emails and phone calls from individuals that want to "help" you. Some may be threatening, others may promise the moon. A few guidelines to remember:
1. If you are looking to do a loan modification you may contact your mortgage company directly to facilitate that. It does not cost anything. A lot of scam artists are charging people thousands of dollars to "consult" on a loan modification without any kind of guaranteed result. This can turn a bad financial situation into a worse one.
2. If you choose to short sale your home all realtor fees are paid by the bank. You have no financial obligation for the real estate professional that sells your home, so choose the best! I've talked with many home sellers that chose to work with "discount brokerages" for a short sale because they were concerned about being able to pay commissions, the result has been a sloppy, ill prepared short sale process that's frustrating and ultimately can lead to sale failure and foreclosure.
3. Keep open lines of communication with your mortgage company! Not only can they not force a short sale, they may have options available for you to help get you back on track.
Best of luck!