What are GMO foods?

Here is a very good article for you to start with.

Here is another link to what scientists found when they studied health effects of Roundup Ready Corn, beyond Monsanto's 90 day study. 

Republished study: long-term toxicity of a Roundup herbicide and a Roundup-tolerant genetically modified maize

They repeated Monsantos finding, but when they went beyond 90 days the inconsequential problems Monsanto identified were CANCER.