Taken from Wikipedia:
"An heirloom plant, heirloom variety, or (especially in the UK) heirloom vegetable is a cultivar that was commonly grown during earlier periods in human history, but which is not used in modern large-scale agriculture. Many heirloom vegetables have kept their traits through open pollination, while fruit varieties such as apples have been propagated over the centuries through grafts and cuttings. The trend of growing heirloom plants in gardens has been growing in popularity in the United States and Europe over the last decade.
Before the industrialization of agriculture, a much wider variety of plant foods was grown for human consumption. In modern agriculture in the industrialized world, most food crops are now grown in large, monocultural plots. In order to maximize consistency, few varieties of each type of crop are grown. These varieties are often selected for their productivity, their ability to withstand mechanical picking and cross-country shipping, and their tolerance to drought, frost, or pesticides. Heirloom gardening can be seen as a reaction against this trend. In the Global South, heirloom plants are still widely grown, for example in the home gardens of South and Southeast Asia, although their future is uncertain.
Heirloom growers have different motivations. Some people grow heirlooms for historical interest, while others want to increase the available gene pool for a particular plant for future generations. Some select heirloom plants due to an interest in traditional organic gardening. Many simply want to taste the different varieties of vegetables, or see whether they can grow a rare variety of plant."