Many lenses carry the word ‘macro' in their title but aren't. To be a true macro lens it must enable you to reproduce your subject at 1:1 lifesize on your sensor. Although you still have great fun shooting close-ups with ‘macro-lite' lenses that only offer half lifesize (1:2) or quarter lifesize (1:4), if you're really into exploring the world of miniature things then there's no substitute for a proper macro lens.
The first thing to know about these lenses is that most are fixed-focal-length (aka ‘prime') optics. The most common focal lengths are around 50mm and 100mm, with some in between, though there are also a few at around the 200mm range. The great benefit of them is that they can also be used for general photography, as they can focus from infinity right down to 1:1 on a single rotation of the focus ring.
One of the main decisions in choosing a macro lens is which focal length to go for. The advantage of the more telephoto macros is that you can obtain your true macro from further away, so if you're photographing shy subjects such as butterflies you don't have to be so close to them. It also means that you won't be casting a shadow over your own subject and have more space between camera and subject for any lighting that you might want to deploy.
Macro photography enables us to reveal the beauty, textures and patterns in very small subjects (or parts of subjects) that we rarely notice when going about our daily lives. Most non-macro lenses won't let you get closer than around half a metre from the subject, which is too far to fill the frame with subjects such as flower petals and insects.
Other Macro Options
Macro lenses are great but there are cheaper ways to obtain true macro with your existing lens, albeit without the same level of convenience and quality. The cheapest option is to use screw-on dioptre lenses. These look like filters and come in a range of strengths, costing from around £25 each. You'll experience a moderate loss of image quality but no loss of functionality.
Extension tubes are spacers that fit between the body and lens, enabling closer focusing to be achieved. Because there are no additional lenses involved there's no loss of image quality, though you will lose up to two stops of light, and depending on the tubes fitted, may lose some functionality. They cost from around £150.