Heirloom tomatoes are grown from seeds that are pollinated naturally and handed down from year to year. Unlike commercial hybrid tomatoes, which are engineered for durability and uniform color and shape at the expense of taste, heirlooms are all about variety and richness of flavor, color, shape, and texture. They offer a way back to that time when tomatoes were seen only a few months of the year, were rarely perfectly round or red—and tasted of summer itself.
You can substitute other tomatoes.
You're more likely to find a variety of them at a farmer's market than at the supermarket and you will pay more for them. Heirlooms are often misshapen and mottled, but this has no bearing on taste. A good heirloom tomato should feel heavy for its size and have an earthy, green scent.
Use as you would any other kind of tomato, but they're best savored in raw dishes.
Don't pile them in a bag; the weight of one will squash another. Leave tomatoes at room temperature until you're ready to use them. Refrigeration causes loss of flavor and a mealy texture.