Probably the most common vegetable in the U.S., tomatoes are actually a fruit rather than a vegetable and they are native to the Americas. It is unclear if the Aztecs actually ate the red tomato we are familiar with today or if the early references were to the green tomatillo. Today there are hundreds of varieties from long, plump San Marzano to the giant beef-steak varieties down to tiny cherry and grape sized tomatoes. In the past few years heirloom varieties have become abundant in grocery stores and farmer's markets.
Sadly, most grocery store varieties are specially bred with thick skins for machine harvesting and are picked green and never develop that distinctive tomato flavor. When possible buy from local growers or grow your own. If you have a freezer, whole ripe tomatoes can be popped into zip lock bags and frozen whole. Although the thawed tomatoes can be sliced and used like a fresh tomato they are perfect for use in soups, stews or sauces.
If you don't have fresh tomatoes then here are some good substitutes:
metric conversions →