Photography is full of well-established beliefs and expectations around the lenses you should own. Need a portrait...
Macro Photography, also known as close-up photography, is the photography of small objects. A macro lens is a lens that offers a magnification of either 1:1, (1x) or sometimes only 1:2 reproduction.
Macro Photography Technical Explanation
Unfortunately, in photograhy the terms "macro" and "micro" and "magnification" are often used inaccurately. Some photographers may use the phrase "high magnification" to describe macro photography, when in fact most macro lenses only reach 1:1 reproduction, or 1x magnification. This means that a 5mm long grain of rice would be perfectly reproduced by the macro lens as 5mm long, when the image hits the camera sensor.
What is Lens Magnification or Reproduction?
Lens magnification is actually a characteristic of the close-focusing capability of a lens, and nothing else. In other words, almost any lens of any focal length could technically be made into a 1:1 macro lens, the only difference would be that a telephoto lens would achieve 1:1 reproduction when the subject is quite a distance away from the camera, where as a medium or wide-angle lens that reaches 1:1 reproduction would have to have the subject mere inches from the lens. There is of course a practical limit to which focal lengths are conducive to 1:1 reproduction, which is why most 1:1 macro lenses are in the range of 50mm to 150mm or 200mm.
Also, optical magnification is independent from a camera's sensor size and resolution. A lens that does 1:1 macro on a full-frame 24 megapixel sensor, is still a 1:1 macro lens on an APS-C 24 megapixel sensor.
However, the resolving power of the sensor (how tightly the pixels are crammed together) does indeed influence the effective magnification of the camera system as a whole. Therefore, having a higher resolution sensor, or a smaller sensor of the same resolution, will allow a macro image to resolve more detail. Unless the lens itself is simply not sharp enough optically, for such a high-resolution sensor.
Macro versus Micro
Most lenses that achieve 1:1 reproduction are named macro lenses, however Nikon uses the term Micro to describe their 1:1 reproduction lenses. This is because of the official definitions of the two words, and how they actually relate to lens magnification or reproduction.
Outside of photography, "macro" means large, while "micro" means small. In the realm of photography and camera lenses, some manufacturers used the term "macro" because they wanted to denote a lens that could make small things appear large, despite the fact that most macro lenses do not exceed 1:1 reproduction, and therefore are not actually making the subject "larger than life", but merely "life-size". Nikon, a company that also makes microscopes, used the term "micro" to describe its lenses that can photograph very small things.
In case you were wondering, there is indeed one lens that can exceed 1:1 reproduction (1x magnification) without any adapters or other accessories, the Canon MPE 65mm f/2.8 Macro, a lens that can ONLY photograph subjects at 1-5x magnification.
Other macro lenses can exceed 1:1 reproduction, of course, when used in conjunction with accessories such as extension tubes or magnification filters.