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Fingerz. Many lenses have a 'Macro' mode it's quite common with many zoom lenses. All that happens is that when you twist the collar to the macro position, all the elements of the lens move further forward ( away from the film plane) and give a much closer closest focusing distance than lenses without the macro setting. It's highly desirable in a lens and gives you the opportunity to tackle many small things in close-up which you wouldn't be able to do without it.
It's not true Macro though, True macro means a reproduction ratio of 1:1 on the sensor or film. In other words a life sized image. A life sized image of that fly (or whatever) on the sensor gives enormous potential for detail once you start to enlarge the image. Some macro lenses are actually capable of producing a larger than life sized image on the sensor, but then it should strictly be called micro photography, but that's being picky and it hardly matters anyway.
True macro lenses are designed purely for the purpose, and are designed to give good detail - even right out to the edges of the frame, which is where 'normal' lenses tend to suffer and why they give their best (sharpest) results stopped down a little in most cases.
The other advantage is you also get a very useful lens for normal photography. My 180mm macro is also a useful, very crisp, mid range tele lens. The 'Macro' setting on lenses which aren't in fact macro lenses, is very misleading and I'm surprised manufacturers keep getting away with it. It's still a very useful feature though and excellent results can be obtained.
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