Considered by chefs as the best paste tomato in the world. Compared to the Roma Tomato, San Marzano tomatoes are thinner and more pointed. The flesh is much thicker with fewer seeds, and the taste is stronger, sweeter and less acidic. Also, unlike the Roma Tomato San Marzano vines are indeterminate and have a somewhat longer season than other paste tomato varieties. As is typical of heirloom plants, San Marzano is an open-pollinated variety that breeds true from generation to generation, making seed saving practical for the home gardener or farmer.
According to Wikipedia, "the first seed of the San Marzano tomato came to Campania in 1770, as a gift from the Viceroyalty of Peru to the Kingdom of Naples, and that it was planted in the area that corresponds to the present commune of San Marzano sul Sarno. They come from a small town of the same name near Naples, Italy, and were first grown in volcanic soil in the shadow of Mount Vesuvius.
In the United States, San Marzano tomatoes are the genetic base for another popular paste tomato, the Roma Tomato. The Roma is a cross between a San Marzano and two other varieties (one of which was also a San Marzano hybrid), was introduced by the USDA in 1955.