If you say 'macro' you might think of something small, in fact it might be something very small, and the use of a macro lens is usually to reflect and portray the small things around us such as insects and flowers. If you look at someone who is crawling in their garden with the camera pointing down towards the flowers; it is most certainly a macro lens he or she uses on the camera.
The term 'macro' means that the lens should be able to reproduce the objects at a true size on the camera sensor, the device that receives the light from the lens and records the picture digitally in the camera. There are many lenses that bears the name 'Macro' but not all can meet this. That is why you should look for whether the lens is capable of reproducing at the scale of 1:1 or not. If they can do that they are to be called 'a true Macro lens'.
Focal lengths of macro lenses can vary and they can be used for other needs than just to photograph flowers and insects. They are often suitable and used for for example portraits. A macro lens is often characterized for its short minumum focus distance and excellent sharpness. Though, something you often read in reviews is that a macro lenses autofocus sometimes is somewhat slower than other lenses of the same kind and price.