HEIRLOOM: vegetables are not a special species of plants. The term heirloom vegetable is used to describe any type of vegetable seed that has been saved and grown for a period of years and is passed down by the gardener that preserved it. It has a provenance, of sorts. To be capable of being saved, all heirloom seed must be open pollinated.
OPEN POLLINATED or OP plants are simply varieties that are capable of producing seeds that will produce seedlings just like the parent plant. Not all plants do this.
HYBRIDS Plant breeders cross breed (NATURALLY) compatible types of plants in an effort to create a plant with the best features of both parents. These are called hybrids and many of our modern plants are the results of these crosses.
GENETICALLY MODIFIED ORGANISMS or (GMOs) which, according to About.com’s Biotech Guide, “can be any plant, animal or microorganism which have been genetically altered using molecular genetics techniques such as gene cloning and protein engineering”. Plants like corn that has the pesticide Bt engineered into its genetic makeup (UNNATURALLY) to make it resistant to certain pests are GMO crops. Bt is a natural pesticide, but it would never naturally find its way into corn seed.