One of the most widely grown fruiting plant in home gardens, tomatoes (Solanum lycopersicum var.) have been bred and selected so extensively that there are now hundreds of varieties in existence. These varieties are further classified by their fruit or growth type, disease resistance and seed origin.
Although exceptions certainly exist, the fruit of tomatoes usually takes one of the following types: the large beefsteak or slicing varieties; the smaller plum or paste-type and pear-shaped varieties; and the still smaller salad tomatoes referred to as cherry or grape. Because tomatoes are such a popularly grown plant in the home garden and so easily hybridized, there are hundreds of varieties that exist within most of these types. There are also cultivars within each type in colors other than the trademark tomato red.
There are two main classes of tomato plant seed origin. Hybrid varieties are developed by crossing two distinct varieties to produce seeds that produce plants with characteristics of both parents. However, the seeds of tomatoes produced by these hybridized seeds do not reliably produce plants that resemble their parents. The other class of tomatoes consists of open pollinated varieties, commonly called heirloom tomatoes. Gardeners can collect seeds from tomatoes grown on these varieties, which will produce plants nearly identical to the plants that produced the seeds. Heirloom tomatoes have been passed down through families for generations, often improving in quality and taste as gardeners selected seeds from the tastiest, healthiest and most robust plants. Botanically this is referred to as "selection," and has been used for centuries, resulting in many of the varieties of all types of vegetables grown in home gardens today.
Tomato plants assume one of two types of growth habit: determinate or indeterminate, with both types existing among hybrid and heirloom varieties. Determinate tomatoes form relatively short, 3- to 4-foot plants that bear their fruits at the terminal ends of their side branches. They tend to produce their crop all at once over a two to three week period, then stop producing. Indeterminate tomatoes grow much taller---up to 12 feet under ideal growing conditions. They produce fruit on small fruiting branches growing off of their side branches. Indeterminate tomato types will produce their crops over a much longer period than determinate types, but tend to begin bearing later in the growing season. This type is ideal for hanging baskets.
A special code next to the variety name of hybrid tomatoes denotes that the variety has exhibited the ability to tolerate or resist common tomato diseases. A "V" denotes verticillum wilt; "F" denotes fusarium wilt; "N" indicates root-knot nematodes; T indicates tobacco mosiac virus; "A" stands for alternaria stem canker; and "S" indicates stemphylium (gray leaf spot). If you have problems with any of these tomato maladies in your garden, your best bet is to plant tolerant or resistant varieties. "Tolerant" means that while the plant may in fact be affected by the disease, it will most likely still produce a crop. "Resistant" means the variety will most likely not contract the disease.