Genetically modified foods have some gene splicing going on in the lab and are made to tolerate things like Roundup being applied to them (won't kill the crop, just the weeds). Sometimes animal materials have been introduced, or in other cases, genes from other plants entirely. They've been altered as to their content, and in the case of Monsanto, many are engineered so that seed saving isn't possible, they're not viable the next year. (This is it in a very slim nutshell). Oh, and Monsanto will sue the hell out of any poor adjacent farmer whose crop is contaminated by the drifting pollen from their GM plants, and will claim that the neighbor somehow stole their patented crop.

SRS