In many cases, the seeds will be viable, and you can grow tomatoes the next year. However, if your seeds came from a hybrid variety, they will likely not produce fruit of the same type as the mother plant (they may have certain characteristics that differ, such as taste, size, color, resistance). Some seeds may even be sterile. To have seeds come true (be the same as the original plant), then you need to save from open-pollinated or heirloom varieties. For more information about this, the basics of seed saving, as well as specific directions on how to prepare and save seeds for tomatoes, you might be interested in the following.
http://www.almanac.com/content/start-saving-those-vegetable-seeds
In many cases, the seeds will be viable, and you can grow tomatoes the next year. However, if your seeds came from a hybrid variety, they will likely not produce fruit of the same type as the mother plant (they may have certain characteristics that differ, such as taste, size, color, resistance). Some seeds may even be sterile. To have seeds come true (be the same as the original plant), then you need to save from open-pollinated or heirloom varieties. For more information about this, the basics of seed saving, as well as specific directions on how to prepare and save seeds for tomatoes, you might be interested in the following.
http://www.almanac.com/content/start-saving-those-vegetable-seeds
(video)
http://www.almanac.com/video/saving-seed-how-save-your-own-seed-tomatoes-peppers-beans-etc
http://www.almanac.com/content/quick-seed-saving-guide-beginners