Macro photography is also called "close-up photography". It is the photography art to produce very fine details of relatively small subjects like spiders, bugs, insects, flower petals etc. When you do macro photography, you basically magnify your close up subjects using the lens glasses. There are some zoom lenses available for the Nikon D90 and other DSLR cameras as well which have particularly close focusing capabilities, making them macro lenses. When you use that type of lenses for macro photography, you may not get the best results when you go more closer to the subject as you can't get more lights into the censor bouncing through the subject. Camera to the subject distance is the most important factor when you are doing macro photography. So in this case, we can have few other alternatives for macro photography. The first one is getting a dedicated macro lens and the second one is to use the extension tube attached to the regular lens. Depending on the budget and the type of camera you have, you might have to go with either one of the two options I just mentioned above.
If the budget is not an issue for you, the best option would be to get a DSLR camera and a dedicated macro lens which fits into the camera. Nikon, Canon and other camera brands provide a dedicated lens for macro photography which is specifically designed to take a close up images allowing more lights into the subject which then passes to the camera sensor through the lens. Such dedicated macro lenses are built with 1:1 magnification capability.
And the second alternative is to use the extension tube which goes between the camera body and the lens. Extension tubes do not have any optical components. It just extends the distance of your lens to the sensor or the film so that you can shoot in a closer focusing distance and achieve a greater magnification. There are different types of extension tubes available in the market. You can use any one of them based on your camera type and the lens specifications.
There are various other cheaper ways to shoot macro as well. For example, you can also use the filters like diopters that goes in front of the lens. I have read somewhere that you can do macro shot with the reverse filters too.
Personally, I think have a dedicated macro (some brand also says micro) lens is the best option to achieve a crisp, clear and magnified details of small subjects which we called macro photography or close-up photography.