Macro photography allows to discover a whole new world of weird, colorful and fascinating creatures, that you often miss by naked eye: it is like exploring another planet! Subjects for macro photography can be found nearly everywhere, and you don't need a particularly advanced camera for macro photography: all SLRs currently in production have enough resolution and relatively low noise (at least at low ISO; the majority of my macro photos had been taken between 50 and 400 ISO). That said, to get good results it is necessary to master the techniques, and to have a lot of patience.
Marbled White - Canon EOS 1Ds Mark III, Canon EF 180mm f/3.5 L USM, 2" f/16, iso 100, tripod. Sesto Fiorentino, Italy.
There are many ways to take macro photos. The easiest solution is to buy a dedicated lens: a "macro" lens can focus much closer than a "non-macro" lens, and usually it reaches 1:1 reproduction ratio. The majority of macro lenses offer very good image quality, high contrast, great sharpness and few or none optical aberrations; often you can get very good results even with 1.4x and 2.0x teleconverters.
Currently, all macros have autofocus, and some have even ultrasonic AF motors (as the Canon 180 Macro USM, the Sigma 180 HSM, the Nikkor 105 VR AF-S), but the focus is relatively slow because the AF has to search the focus between a very wide range of distances; you can get a slightly faster AF using the Focus Limiter, that reduces the AF range, or pre-focusing manually (thanks to Full Time Manual Focus you can switch from AF to MF in every moment). That said, the 99% of my macro photos is taken in manual focus, so I really don't care about AF in a macro lens.
The focal length is very important. Macro lenses range from 50mm to 200mm; even though both a 50 Macro and a 180 Macro reach the same magnification (1:1), the longer lens gives a much more out-of-focus background and more working distance
. When you look at the specifications of a macro lens, you have to pay attention both to the focusing distance and the working distance. The focusing distance, as you already know, is the shortest distance where you can focus: but this is not the actual distance between you and the subject, it is just the distance between the focus plane (the sensor) and the subject. In macro photography, it is very important to know the working distance - that is the distance between the front element of the lens and the subject. For example, with a 50mm Macro you have a minimum focusing distance of 19 centimeters, but actually the distance between the front element and the subject is just 7 centimeters, and if you mount the lens hood, this distance is further reduced to 4-5 centimeters. As you can imagine, it is not easy to get so close to a butterfly or another animal: this is one of the reasons to prefer a long lens for macro photo. The following table lists the focusing distance and the working distance for the most common macro focal lengths.