What exactly are heirloom seeds?
To keep this answer simple, heirloom seeds are the seeds of plants which have been saved from generation to generation. Heirloom seeds come from plants that are open-pollinated. Open-pollination is a natural occurrence where plants are pollinated from uncontrolled factors such as animals (deer, hummingbirds, bees, etc.) and weather (like wind or rain).
Usually varieties referred to as heirloom have been around for more than 50 years and offer a wonderful history in the names and flavors that continually provide dependable quality season after season. Not all seeds are heirloom seeds. If they are, then they will be marked as such.
The seed from heirloom plants can be saved at the end of the season to be used again the next year!