Although many of us would have it differently, tomatoes are only in season for a portion of the year. The rest of the time, they're either scarce or expensive. Well-prepared chefs often choose to can some of their summer crop for use in off-season months. Canning tomatoes is surprisingly easy and affordable, and doing it yourself eliminates exposure to the resin and chemicals found in some commercially canned tomatoes.
The best tomatoes for canning will be full of flavor, meaty and not so big that they can't fit through the opening of the jar you're using (you can just as easily can chopped or halved tomatoes, though). Start by boiling jars for about 15 minutes to sterilize them. Put tomatoes in the jars, taking care not to pack them too full. Then add lemon juice at a ratio of about two tablespoons per quart of tomatoes. Doing so helps keep them from spoiling. Re-boil the sealed, filled jars to complete to sterilization process, then save them to eat at your leisure.