Colic is the generic name for any stomach pain, and is usually attributed to changes in diet, or the horse finding and eating food it shouldn't have. Here is a good link with good information: http://extension.oregonstate.edu/catalo … ec/ec1474/
Preventing colic is very important, as surgery can be extremely expensive and there isn't a good chance of the horse recovering well afterwards, although there are other treatments, if it gets too bad then surgery could be the only option.