There are some lenses that are built specifically for Macro, like the one Dalantech uses which is the MPE-65, I think.
That can only be used for Macro shots. There are lenses like the 100-300 you're talking about and the 100mm Macro lens that is so popular. These lenses can be used for regular photography, but they also allow the lens to focus very close to the subject.
Sometime if you are going to use the lens, depends on which model you have, you might have to click a button which enables the lens to focus at short distances, other models will just automatically focus very close without switching anything.
Most lenses dont have this ability and many will have a minimum of a few feet focusing distance. Go try and focus your 100-300 within a few inches and if it allows the lens to focus, than you have a macro lens. I dont think you'll get images like Dalantech's, but you should be able to get some very nice flower shots or upclose images of what ever subject you're trying to take pictures of
Make more sense now?