There are several different approaches to GMO crops. One, like you say, is the herbicide-ready crop. Another is the insecticidal crop, like Bt-Corn, that produces insecticides that kill common pests, and works in the way that the other poster suggests. Yet a third is to impart nutritional characteristics to a crop that weren't there before, like vitamin-A-producing rice. Of course, there are also the usual goals of higher yield, drought resistance, reduced waste, ease of harvesting and regularity of product. The hybridization and selective breeding processes have done the majority of work in those areas and are not considered GMO by most activists, but they are nonetheless modified.

it's almost like someone at Monsanto thought this approach might allow them to sell 5000 times more chemicals too. pretty lucky cooincidence they got there after all that science happened to show them that this is the most effective approach to solve world hunger (how is that going btw?)