|Basic definition of heirloom ~ a variety of vegetable (or flower, etc.) that, for generations and generations, has been grown from seed saved from the previous year's crop. Many people will define it differently though, but that's a generally accepted definition. All heirlooms, when defined this way, are open pollinated ~ that means that if you take measures to ensure no cross pollination with another type takes place, the seeds you save from them will grow true (the same as the fruit/flower you took the seeds from). Think of dogs ~ if you cross a Dalmation with a Dalmation, you get white puppies with black spots, but if you cross a Dalmation with a Lab ("cross pollination"), you'll get all sorts of compilations of the two types.|
Hybrid ~ a cross between two different varieties. This can be either an accidental cross in your garden that, when grown out, will produce who knows what OR a purposeful cross between two varieties known to produce a certain distinct variety (almost all plants grown from seeds of that cross will grow the same type). This last one is the one you get when you buy hybrid seeds ~ the seed companies have intensively studied and bred the parent plants to ensure that when they cross them, the offspring will all be the same (if you want to know how/why, let me know ~ it's a bit of a deep subject for just a quickie answer here, simple but takes a while to explain).
Genetically modified seeds (GMOs = genetically modified organisms is the technical term) are ones made by splicing in genes that would never have naturally been able to be put into a plant's genetic code. Like Bt corn ~ the gene giving the caterpillar-killing ability of the Bt bacteria was taken from the bacteria and spliced into the corn's genes in a lab. That would never happen in nature.
Most (I think all actually) seeds available at this time to general consumers are not GMOs. To grow those requires a contract with the company holding the patent on them, so due to the cost of drawing up, signing and enforcing that contract, the seed-producing companies aren't likely to do it for a $3 pack of seeds. GMOs are aimed at farmers who buy large quantities of seeds, large enough to make it worth the contract hassle.
So, the seeds that you and I buy in the little packets are produced by cross breeding, not gene splicing.