18mm is at the wide end and 200mm is at the long ends. It is not correct to say that 500mm can zoom better than 200mm because by just stating 1 figure (500mm or 200mm) it would means the focal lenght is merely 500mm or 200mm.
There is no such thing as better zoom... because all lenses are designed for specific uses. And so there is no better or worst.
For a starter, I would suggest you search the net and read a bit more on the focal length.