There are many other varieties, all with different characteristics and requirements. Many will have a group of letters beginning the variety name. Most hybrids were created to blend the favorable traits of parent plants. For example, one may produce high yields and another strong disease resistance. Plants are also crossed to recreate traits such as height, fruit size or flower color.
Hybrids are usually more vigorous, productive and uniform in growth than nonhybrids.
On the down side, they can't be reproduced from seed. If you save seeds from this year's fruits, the results will be entirely different plants. Tomato varieties coded with these letters are less susceptible, but not immune, to those problems. Always look for varieties that are resistant to diseases. This will reduce your work and increase your garden yield.