Technically, an "heirloom tomato" is one grown from seeds passed down from generation to generation.
There are a lot of tomatoes at the Oak Park Farmer's Market that are designated as "heirloom," and it's clearly a very popular type of tomato.
But there is no standardized definition of "heirloom," and so it's pretty much in the same category that "organic" was in twenty years ago. It's a marketing word that apparently does not require any objective substantiation.
The word "heirloom" was used the first time in 1981 by Kent Whealy, of Seed Savers Exchange, a fine organization we've discussed before.
Cruising around the Farmers' Market, I saw Early Girl, Black Krim and Green Zebra tomatoes, all grouped under the rubric "heirloom."
But the weird thing is, Green Zebra, to take just one example, is not a seed passed down from generation to generation. The Green Zebra was bred in 1983 by Tom Wagner in Everett, Washington. Wagner introduced the Green Zebra in his Tater-Mater Seed Catalog, and although it's a meaty and beautiful tomato with a lightly tart finish, it's not really an heirloom even by the general definition given above.
We grow heirloom tomatoes exclusively in our garden. They look interesting on salads, and the taste is a little different than the standard Beefsteak, but for overall balance of acidity and sweetness, and gorgeous appearance, I have to hand it to the Beefsteak and also its cousin, the Brandywine, which is a true heirloom.
Different colored tomatoes, just like different colored Bell peppers, seems to taste different. And perhaps it's force of habit, but I prefer the big red ones, which in addition to great taste, also look every bit as great as their green, yellow and purple brethren. Actually, if mere color is the main consideration, I think they look better with lettuce (more contrast).