Usually, a dedicated 'macro' lens is one that allows at least 1:1 reproduction. "1:1" means the subject is the same size on your sensor as it is in real life.... a 5mm-long ant will cast a 5mm-long image on your camera sensor. "At least" because some lenses can do even better, e.g. "2:1" means a 5mm-long ant will be 10mm on the sensor. Try this link:
http://en.wikipedia.org/wiki/Macro_photography
A "zoom" lens is one that has a range of focal lengths, e.g., 35-70, 70-200, 17-35. This is in contrast with a "fixed focus" lens (also called a "prime" lens) which only has one focal length e.g. 35mm, 50mm, 85mm. Try this link:
http://en.wikipedia.org/wiki/Photographic_lens
Some "zoom" lenses will be labelled as "macro". What it usually means is that they will allow you to focus very near to your subject, but they will usually not give you 1:1 reproduction of a dedicated macro lens. For example, Sigma 70-300 f4-5.6 MACRO. Minimum focusing 150cm, but 95cm in 'macro' mode. Magnification 1:4.1, but up to 1:2 in 'macro' mode. Quite good, but still not 1:1. ;p And image quality of a dedicated macro lens, shooting at macro range, is almost always better than image quality of a non-dedicated lens.
Hope this helps.