Heirloom is a term that is being thrown around in the produce business with more and more frequency in the last few years. its can be as confusing as the terms "organic", "sustainable", or "local". wikipedia calls an heirloom plant a style of produce that isnt produced in large scale production (not sure i agree as i can buy heirloom tomatoes at my local megamart), it also says that it has to be open pollinated, and that it is a style that was common "earlier in human history". i have also heard that to be considered a true heirloom the style of cultivar has to be at least 100 yrs old. the growers and sellers of most heirloom varieties would love you to believe that it also means the product is organic, and sustainable but the terms do not have to overlap. they can grow heirloom potatoes and spray them with pesticides every day, and then have the gaul to attempt to stump you at the grocery store.
clarification-up until the early 1900's carrots were never really orange. they were white, purple, and grey. now someone got into trying to cultivate them in a more healthy way, and then someone took those and bred them to be more attractive, and some else figured out how to hybrid those plants and fortify them with vitamins, then someone hybrid that and years later we all assume the carrot has always been orange. its just how it works. all sorts of grafting, pollinating, engineering, and science is involved. if you think about it all of our food has some sort of wild starting point, and a whole history of how it grew to look like it does when you see it in a grocery store, that can span thousands of years. the goal for growers and chefs is to expose us to that history. part of the problem is that farmers throughout history never really took notes or wrote books about when they grew what, and how. so some "heirloom" variety of garlic could've been eaten in our local area in the earliest days of America, or somewhere else across the world as far back as time itself could recall.
most popular i would think would be the heirloom tomato varieties that are becoming a staple at restaurants, farmers markets, etc. there are thousands of varieties, some absolutely gorgeous, others absurdly ugly. but as mentioned garlic, carrots, potatoes, celery, salad greens, braising greens, apples, peaches, watermelon (had the chance to work with yellow hermiston watermelon, and a russian moon and stars watermelon just this week-sorry no pictures) and plenty more are all things i have seen badged with an heirloom tag.
we are in the middle of (locally at least) harvesting the best our soil has to offer. looks can be deceiving, so don't be afraid to pickup some oddly colored produce in your own neck of the woods. cooking will almost always be the same as their modified relatives that you are used to. in the restaurant we use things like this for the exact same reasons you would use them at home. they are a wonderful conversation piece for myself, cooks, servers, and customers. they also have an extremely valuable presentation. at home, especially with children, they will help to learn a great lesson, and start a conversation that will plant a great seed, and impress your family or guests.