I have a Canon 550d, and intend to take insect/flower closeup/macro shots. But I am not sure what the difference is between the Canon 18-135 lens vs a Canon 100mm Macro lens in terms of their macro capabilities. I mean, why wouldn't the 18-135mm (at 100 mm) be equivalent to the 100mm macro lens?
The problem is minimum focus distance. A lens normally can't focus on something really close to it. Your 18-135 lens can only focus on things that are around .4 meters away. The 100mm macro on the other hand can focus on something at .3 meters. This may not seem like a lot, but, it is enough that the maximum magnification of the lens is 1/4 that of a true macro lens.
In other words, without a macro lens, you have to take the photo from further away and thus the object is much smaller in the frame. A macro lens lets you get closer to the object and make it larger in the frame.
Short answer is you won't be able to get close enough to your subject to produce a 1:1 replica of the subject on your camera's sensor.
While both lenses might hit that 100mm angle, only a macro lens will allow you to get a few inches from the tip of the lens to your subject.
Bear in mind the minimum focal distance is from the subject THROUGH the lens to your sensor plate. On Nikons this location is indicated with a circle with a line through it on the side of the camera. Not sure about Canons.
Back to that 1:1 bit, this means the image on the sensor is the same size as the object in real life when taken at the minimum focal distance from the subject. Then when you view the image on your screen, it becomes huge.