An onion is a bulb. They belong to the allium family, which is also home to edible plants such as chives (also known as scallions), garlic, leeks and shallots.
As an onion begins to grow, the plant sprouts hollow leaves that then turn into edible bulbs. As the green leaves grow, they fall over towards the ground as the bulb of the onion keeps growing. The longer the leaves of the onion are allowed to grow, the larger the bulb becomes. Once the onions are ready for harvest, gardeners can remove the green leaves and dig the onion bulbs out of the ground. This is better than pulling, which can detach the onions from the stems and keep them from lasting as long out of the ground.