While you may have noticed that the extreme cyclists have killer calves, regular biking works most muscles in the body. Each time you hop on that bike, your abdominal and back muscles work to stabilize your core, while your shoulders and upper arms work to support your body at the handlebars.
The circular pedaling motion also strengthens your hip flexors (the muscles below the hipbones), quadriceps (front thigh muscles) and hamstring (back of the thigh), and, to a less extent, your butt.