It's firmly strapped against earthquakes, and up off the floor where it won't ignite fumes. If it's inside, it's sitting on a drain pan so that if anything does leak, it won' t flood your living room, or perhaps your garage. The plumbing around it is in good shape, and water isn't dripping onto it from overhead plumbing.
Gas can be dangerous.
Water pressure, if not dangerous, can cause household disasters. There is considerable information below on how to work on your water heater, however, you are responsible for how you use it. If you have the slightest doubt of your ability, get a professional. People grump about what plumbers charge, but what you're paying for is a truckload of specialized tools and someone with the expertise to use them if things don't go right. That often happens with plumbing.