A macro lens is one that's *capable* of making a subject ( say a bee ) the same size on the sensor ( film, CMOS, whatever ) as it is in real life. This is also called 1:1 magnification, or life-size. But the word "dedicated" is kind of a faux-pas, as macro lenses for 35 mm are also capable of focusing to infinity, like any other lens. Which is why so many people use Canon's 100/2.8 Macro for portraits, and as a short telephoto lens.

A fact that people should be aware of, is that a decent copy of a macro lens for 35 mm format, is among the best lenses available for the format. Just like 50 mm primes. In other words, as far as sharpness/optical quality is concerned, it makes little difference whether you use Canon's 180/3.5L Macro, or a Sigma/Tamron/Quantaray, or whatever other macro lens your heart ( and bank account ) desires. The differences between these lenses come into play mostly when you focus considerably further out ( like in a portrait ), and in things like the focus system, flare resistance, bokeh, and so on.

Okay, getting back to the magnification thing ... a *true* macro lens can focus to 1:1, which means on a Rebel D ( which has a 23 mm wide sensor ) you can fill your frame with a subject 23 mm wide. Canon's 50/2.5 Macro isn't a true macro lens; it gets to half life-size, or 1:2 magnification. That means you can fill your frame with a subject 46 mm wide, but getting closer means tubes, dioptric filters, or cropping in post. Sigma makes some supertele lenses that happen to go to 1:3, meaning you can fill the frame with a subject 69 mm wide. When you evaulate a lens for macro work, you need to look at this 1:X figure; that 50/2.5 gives more magnification at 1:2 than my 300/4L IS at 1:4; the shorter focal length just means you have to stand six times closer to get it.

Now, I use my 300/4L IS as my macro lens, a good deal of the time. I'm shooting insect macros, which I believe is what this question was originally about. While 1:4 isn't enough magnification a lot of the time, the lens is sharp enough to crop down, and the working distance a 300 mm lens gives me makes all the difference between getting a photo, and scaring an insect off. ( The 75-300 IS also gives 1:4 magnification, by the way. )