Most fruits have natural acids that help inhibit the growth of microorganisms, which means it's safe to can them in boiling water.
Vegetables, however, are low in acids; they're much more likely to support the growth of harmful bacteria and molds. (One especially dangerous bacterium, Clostridium botulinum, doesn't need oxygen to live and can survive in a sealed jar even after 5 hours of boiling.) To achieve the necessary higher-than-boiling temperatures, you must process vegetables in a pressure canner.
Always inspect a home-canned jar carefully before serving its contents. If the jar has leaked, shows patches of mold, or has a swollen lid, or if the food has a foamy or murky appearance, discard the food and the jar. The odor from the opened jar should be pleasant. If the food doesn't look or smell right, don't use it.