OCA has seen quite a few questions on this topic. There are a number of causes of bloat but for the most part they can be categorized into either physiological, physical, biological or nutritional.

Physical Bloat:

An example would be an obstruction in the esophagus not allowing the animal to ruminate off the rumen gas.  Another would be an animal that has cast itself and can't get into sternal recumbency.  In these cases the bloat comes off quickly once the cause is corrected.

Physiological Bloat:

In this instance there is a problem with the innervation of the stomach(s) in that is not allowing it to function properly.  Hardware and vagus indigestion are the 2 more common diagnoses of this type.  In these cases the bloat has a poorer prognosis as the animal does not improve with treatment or relapses at a later date.

Biological Bloat:

Is seen in cattle (particularly calves) with an overgrowth of Clostridia bacteria in the digestive tract.  This type is greatly correlated with the diet but usually occurs after some insult to the intestinal tract.  It could also be due to a parasitic overgrowth seen in calves >3wks that are more pot-bellied than bloated.  These cases are treated by treating the potential pathogens in the intestines.

Nutritional Bloat:

Bloat caused by the diet.  Examples are grain overload, frothy bloat (seen on lush alfalfa pasture).  It can also be seen when 2 foodstuffs are mixed such as milk or alfalfa hay, and that can develop into a biological bloat.  These cases need to be treated with anitgas products and a change in the diet.

Passing a stomach tube is the first step in determining what kind of bloat it is.  Is it free gas(physical,physiological) bloat or frothy (nutritional, biological) bloat?

With a free gas bloat,  the gas comes off with the passing of a stomach tube usually rather quickly.  The contrary occurs in a case of frothy bloat, where the bloat is comprised of many tiny bubbles that won't form a bubble large enough to be eructated or burped off.  In these cases, an antigas product needs to be used.