Any one of the several seeds contained within a ripe tomato has all of the genetic material it needs to produce a new plant. In order to maintain its biological diversity, however, the seeds need to be distributed at some distance from the parent plant. This is true of all plants, and the means of dispersal varies widely among the different species. Some are spread by the wind, some by water, some with sticky coatings or spikes get stuck to the fur of passing animals, and some get eaten by animals to be later passed, undigested, onto the ground great distances away.
Tomatoes, along with many other fruits such as strawberries, raspberries, grapes and plums contain seeds surrounded by gelatinous coats which allow the seeds to pass through an animal's digestive system without damage to the seed itself. This is a biological process called endozoochory. The fruit is consumed by a bird, or perhaps a mammal, along with the seeds. The seeds themselves are later excreted without being digested and are thereby scattered. In time, through the action of natural processes in the soil, the outer coating dissolves away, leaving the seed free to germinate and produce a new plant.
Tomatoes undergo open pollination. Nearly all of the genetic characteristics will remain within the seed because tomatoes are, to a very large extent, self-pollinated. Any tomato variety that either self-pollinates or becomes cross-pollinated by other tomato plants of the same variety will produce seeds which bear the genetic material of the parent plant. Other tomato varieties are hybrids.
Hybrids are the product of cross pollination between two different tomato varieties. The process of creating a hybrid combines the traits of the two parent plants. Hybrids are often created deliberately to produce tomatoes that are particularly tasty, or disease resistant, or are extremely productive. Plants grown from hybrid seed will not, however, be identical to the hybrid parents once they are dispersed. Rather, they will exhibit some random combination of the traits of the two parent plants that were initially crossed.
It is not always possible to forecast the qualities that the fruit of a hybrid will develop. Experimenters can work with hybrids to isolate the desired characteristics and ensure their continuance but, in nature, it remains the province of the birds to eat the fruits and scatter their seeds for future generations.