A macro lens is a lens that specifically allows you to focus extremely close to a subject so that it appears large in the viewfinder (and in the final image). "True" macro lenses are able to project subjects onto the camera's sensor life-sized at a 1:1 reproduction ratio resulting in a 1.0x MM (Maximum Magnification) at the lens' MFD (Minimum Focus Distance, measured from the subject to the sensor), meaning that a 0.6" (15mm) long subject would be projected 0.6" (15mm) long onto the sensor. While that doesn't sound like a big deal, keep in mind that a subject measuring only 1.4 x .9" (36.0 x 24.0mm) will completely fill the frame of a full frame DSLR. When viewed on a large display, tiny details in your subjects become conspicuous features while using true macro lenses.
A select few macro lenses actually exceed life-sized reproduction (the Canon MP-E 65mm 1-5x Macro is a notable example) for the ultimate in close-up DSLR photography. Besides true macros, lenses featuring a MM between 0.50x and 1.0x can also be considered macro lenses as they permit significantly closer focus than typical lenses provide.
Macro lenses come in a variety of focal lengths from wide-angle to telephoto. The important thing to keep in mind is that the lens' focal length will determine your field of view, working distance available and background blur capable when photographing your subject. For instance, the Canon EF-S 35mm f/2.8 Macro IS STM features a 35mm focal length (as the name implies) and produces 1.0x magnification at its MFD of 5.1" (130mm). In contrast, the Canon EF 180mm f/3.5L Macro USM produces the same magnification with a subject placed 18.9" (480mm) from the sensor. The extra working distance provided by longer focal length macro lenses can be very beneficial if photographing skittish subjects such as insects. Longer focal lengths also produce a more pronounced background blur which can be great for showcasing your subject through isolation.
On that note, let's consider the many common uses for macro lenses. Typical macro subjects include the aforementioned insects, flowers, jewelry (especially engagement and/or wedding rings), coins, and everyday small objects. If your subject isn't necessarily small, you can focus on the small details of a larger subject to create intriguing macro imagery.
Now that you know more about macro lenses, you may consider adding one to your kit. And on that note, checking out our Macro Lens Recommendations will help you find the right macro lens for you.
Now get out there and enjoy shooting our big world full of small things!