Also called Gastric Dilation Vovulus, bloat refers to the condition in which the stomach of a dog swells up with gas and fluid.
In extreme, more dangerous cases of bloat the stomach can actually rotate 180 degrees or more cutting off the esophagus and other ways for the dog to get relief from gas and fluids.
The blood supply to the intestines can also be cut off by this twist. 
What causes Bloat?
These larger breeds tend to ‘inhale’ their food, and larger amounts of dry kibble get into their stomachs and begin to absorb water and other gastric fluids. These fluids cause the kibble to swell which in turn causes the stomachs to swell.
Rigorous play after eating a large meal can also increase the likelihood of a dog becoming bloated. [1,2]
What are the symptoms of Bloat?
Symptoms of bloat include excessive salivation, abdominal swelling, attempts to vomit and defecate, pain when pressure is applied to the stomach area. In extreme cases of bloat when the stomach has twisted, the dog’s breathing will be rapid, its heartbeat will increase, mouth membranes may be cold and pale, and the dog might even collapse.
In instances such as these rush your dog to the veterinarian IMMEDIATELY! 
How can I know for sure my dog has Bloat?
It is difficult to diagnose bloat simply by examination. However, if your dogs stomach is swollen and he/she appears to be in pain, it is best to take him/her to the vet so that proper treatment can be provided. 
How can I treat Bloat?
Bloat is a serious and life threatening condition. If you suspect your dog has bloat every effort should be taken to get your dog to the vet. Once there your veterinarian may try to relieve gas pressure by passing a tube down the dogs esophagus to its stomach.
If the stomach has twisted this method may not be effective and surgery will be necessary.
Your vet will have to perform an emergency operation to save your dogs life in which he/she will have to untwist the stomach and anchor it (gastroplexy) to prevent reoccurence. 
How can I prevent Bloat?
Preventing bloat is not too terribly difficult. If you have breed of dog that is predisposed to bloat it is recommended that you avoid large daily feedings and instead feed the dog small amounts of food a couple of times a day.
Soak the food in water or broth for 20 minutes or so before feeding to avoid extra water absorption and swelling in the stomach.
Limit water intake for 30-60 minutes after feeding your dog.
Keep them calm after eating – many breeders suggest crating them for an hour or so after meal times.
Also feeding your large breed dog on a raised surface will prevent them from gulping down too much air. 
More sites dealing with Bloat: