Unless you have a very sharp knife, cutting tomatoes is a messy affair. A serrated knife does better than most other knives.
Photo by epSos .de.
Over at The Kitchn they explain why a serrated knife is easier:
While you can cut tomatoes with a chef's knife (in fact, knife sharpeners sometimes use tomatoes as a test), your blade has to be ultra-sharp to do a good job. Because tomatoes have thin skins but soft, delicate flesh underneath, anything less than sharp won't get through the skin easily. You probably also have to apply a lot of pressure, running the risk of crushing the tomato.
But a serrated knife with all the little teeth? Those teeth grab onto a tomato's thin skin and cut straight through rather than slipping off the side or squishing the tomato with unnecessary pressure.