"I'm unclear as to why some people vaporize weed and other people smoke joints. Is there really a big difference between the two?"
A vaporizer is a device that controls the temperature and disallows medicine reaching the temperature at which combustion (fire) occurs. The vaporizer temperature becomes hot enough for the cannabinoids in the medicine to go from solid or liquid to vapor state. There are a variety of vaporizers on the market and shredded flower, oil concentrates, and dab products (e.g. wax) may be vaporized.
Any time a flame source is used to ignite medicine, whether it be rolled in paper, placed in a pipe, or used with a water pipe/bong, the product is burned and smoke is created. Smoke contains carbon monoxide, tar, and carcinogens which may be harmful to your health.