There are hybrids, heirlooms and open-pollinated seed. I have a favorite hybrid tomato that I grow every year called Jetsetter, I have to purchase the seeds because I don't save the seeds from it. I grow many other tomatoes that are open-pollinated or heirlooms and I save the seeds from them to replant again next year.
Here are a couple of links that might help:
http://www.grit.com/farm-and-g...
http://aggie-horticulture.tamu...
