Bt corn is a variant of corn, genetically altered by methods of biotechnology to become poisonous for insects. This was achieved by inserting a gene from the soil-dwelling microorganism Bacillus thuringiensis (thus Bt) into the corn genome. This gene codes for a toxin that will crystallize in the digestive tract of insect larvae, leading to its starvation.
While announced as an outstanding example for "green" biotechnology by the industry, a study from Cornell University in New York suggested that Bt corn might be poisonous to monarch butterfly larvae. Later studies showed that the extreme conditions of the study could not really occur in nature. (I think this is correct? Anybody?)