Safe canning depends on processing foods at a high-enough temperature for the correct length of time.
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.