Here's the low-down from Wikipedia:
Abigail "Abby" Loraine Hensel and Brittany Lee Hensel (born 7 March 1990, Carver County, Minnesota, United States), are dicephalic conjoined twins. Brittany is the left twin, and Abigail is the right twin. They have two spines which join at the pelvis. They have two stomachs, three lungs, and two arms. (A third, underdeveloped and useless arm between their heads was amputated in infancy.)
Each of the twins manages one side of their conjoined body, controlling her own arm and leg, and receives no sensation from the other side. Yet, by coordinating their efforts, they are able to walk and run normally. They enjoy hobbies and sports including volleyball, basketball, kickball, swimming, cycling, and singing. They also play the piano and are avid computer users. Like many teenagers of their generation, they enjoy softball, digital photography, internet, social networking, and talking on the telephone. In school, both twins love to talk, though Abby is more extroverted while Brittany is introverted.
Common organs
Most of Abigail and Brittany's shared organs lie below the waist line.
2 heads
2 arms -- originally 3, but the short malformed central arm was removed
2 spinal cords and backbones which merge at the pelvis -- surgery corrected scoliosis
3½ lungs -- surgery expanded their chest cavity
2 breasts
2 hearts in a shared circulatory system -- medicine taken by either affects both
1 liver
2 stomachs
3 kidneys
2 gallbladders
1 bladder
1 ribcage
1 large intestine
1 female reproductive system