The common onion (Allium cepa) is a biennial bulb that is closely related to garlic (A. sativum), shallots (A. ascalonicum) and chives (A. schoenoprasum). Onions have hollow, tubular, blue-green leaves that emerge from a bulb, which is actually a modified leaf structure with many layers. A shallow network of roots extend from the bottom of the bulb and the bulb might push partially above ground as the plant matures.
Onions should be planted in the spring and they have a moderate growth rate. They can be planted from seeds, transplants (seedlings that have just sprouted) or sets (small onion bulbs that are about to begin their second, final year of growth). Note that onions are toxic to pets due to their chemical compounds, so be mindful about where you plant them.