There are several different types of onions, most all of which are used to enhance the flavor of food when cooking. The different fresh types are mostly recognized by their color. They are divided into two main categories: green and dry. Dry onions are further categorized as either spring/summer or fall/winter. The spring/summer varieties are sweet and the fall/winter varieties are for storage.
Green onions are those that are harvested while their shoots are still young and green. They can be chopped and used for toppings on salads, baked potatoes, soup, and a wide variety of other culinary uses. The green onion and the scallion are typically used interchangeably both in reference to the onion itself and in cooking, but they are actually two different varieties.
Dry onions can be red, white, or yellow in color. They are harvested once the shoots have died and the onions are left with a paper-like covering encasing the fleshy vegetable. Spring/summer varieties are the sweeter type, but do not store well like fall/winter varieties. The most common sweet onion is the Vidalia, which is named for their growing location in Georgia. Others include the Sweet Imperial, the Spring Sweet, and the Walla Walla, named for its growing location in Washington.
Storage onions such as the red onion, the boiling onion, the shallot, and the Spanish onion are found in stores year round, but are at their prime in late fall. They are most often yellow, though they can come in all three common dry colors. They have a longer shelf life than spring varieties and contain less sugar and water. This makes them ideal for flavoring dishes that require long simmering or cooking times such as chili or roasting meats.
In addition to flavoring many dishes, onions are also prepared pickled, boiled, sauteed, and deep-fried. Those that are deep-fried can be prepared as rings or petals are a popular appetizer in many American restaurants. Besides fresh, they can also be purchased canned, frozen, and dehydrated as a spice or powdered.
When selecting fresh onions in the store, look for those that are firm and dry with a shiny, crackling outer skin. Be aware of excessively strong odors as this might indicate rotting. Rather, fresh whole onions should have a mild odor. Also avoid those with dark spots as this can indicate mold. Sweet onions are best used fresh because they do not keep well. To get the best flavor, purchase and use them within a day or two.