The genus of Alliums contains the members of the onion family, as well as including a few plants that aren't normally eaten. Their growing habits and appearance are similar in many ways, and people would recognize them despite the fact that few kinds are actually available in the store.
Even large supermarket chains in the US seldom sell more than three varieties of onions, two kinds of garlic, chives, leek, and shallots. It is important to understand that there are over 1200 more species of alliums, most of them growing wild in various lands and climates.
Alliums have several things in common, regardless of where they grow. All of them have bulbs, though in a few the bulb isn't well developed, such as in leek. In others, like garlic, the bulb is multiple. All of them are also fragrant, primarily due to sulfur compounds. These sulfur compounds also give the members of the genus their flavor, but which can cause a person's eyes to water when they are cut or chopped.
All alliums also have long green leaves, generally hollow, that are normally edible if the bulb is also edible. The length of the leaves is highly variable, from a few inches (around a centimeter) to several feet (about a meter) or more. The leaves are unbranching and arise from the bulb.
There can be considerable confusion between species in the wild. For instance, in a mountain meadow containing wild onions and wild chives, the bulbs, aroma, and leaves are so similar that most people would find it difficult to distinguish one from the other. Doubt is removed when they flower, however. Wild onions usually have a white or pinkish blossom while that of wild chives is a vivid purple.
In the kitchen, allium vegetables can nearly be interchanged with one another, though each has a slightly different flavor. For example, the leaves of green onions can be used in the place of chive leaves very effectively. Many of the best dishes use a combination of different kinds of allium vegetables. Each adds a slightly different flavor, especially to sauces. This can easily be seen by preparing and tasting spaghetti sauce using chopped yellow onion and garlic, then trying it after adding red, green, and white onions, perhaps with some chives and leek.
It should be mentioned here that the green onions sold in the store are usually not a separate species of onion. They are simply immature onions, harvested primarily for the leaves. In addition, many varieties are named after the region where they are grown, rather than because of species differences. A "Walla Walla White Onion" is famous in the Pacific Northwest, and is named for the area in Washington State where they are grown. They are usually sweet and juicy, but not especially different than "Bermuda" whites, botanically.
Where and how they are grown does have an impact on flavor, however. Hotter areas with less water usually produce hotter onions.
Though harder to come by, wild onions, garlic, leeks, and chives all tend to have a better and fuller taste than their domesticated and more commonly grown cousins.
Growing members of the allium genus is usually simple, since they require far less care than many other vegetables, and are more adept at growing in poor soils than many other plants. The allium vegetables even have similar medicinal value and are used for a number of different ailments, including colds and coughs. Again, this is due to the sulfur compounds contained in the plant. Some species are faster acting than others, because the amounts of the compounds differ from species to species.
Storage methods differ for different kinds of these vegetables. For instance, onions and garlic store well in cool, dark, dry places. Leek, green onion, and chives can be refrigerated. However, nearly all of them can be chopped, dried and stored for later use in cooking.
The allium vegetables are a large group, and the pity is that most people won't be able to sample the flavor variations between most of them. Many more people won't sample those found in the wild, so they will never know what they are missing.
One thing is for certain, the allium group of plants has and will continue to have a culinary and medicinal impact throughout the world.