If the cracks are forming as concentric rings around the stem, they may be a sign that the fruit has become too ripe before it is picked. Pick all fruits as soon as they ripen to prevent cracking.
If the cracks on your tomatoes are running from the stem down the sides of the tomato, they may be caused by a number of different fungi. Try keeping the fruits off the ground and minimize surface watering. It is better to use a furrow or drip irrigation system rather than a sprinkler system to water tomatoes.