There are two major types of polycystic kidney disease (PKD).
Autosomal dominant PKD is the most common form; 90% of the people diagnosed with PKD have this version. It is a genetic disease (inheritable from a single bad gene from either parent) which is characterized by the development of abnormal sacs of fluid (cysts) in the kidneys and by high blood pressure. The cysts cause back and side pain, blood in the urine, and frequent kidney infections. Other symptoms include kidney stones, and diverticulitis. People with PKD may also suffer from cysts in other organs such as the liver, intestines, brain, pancreas, ovaries, and spleen. If the brain is affected, the person may suffer an aneurysm. The disease may appear when the individual is in his or her late teens, but symptoms usually begin between the ages of 30 and 40. About half a million people in the United States have this type of PKD.
Some people suffer from severe versions of this disease that ultimately cause kidney failure, but others have only mild versions that only cause minor problems; most people with this form of PKD lead normal lives.
The disease is generally diagnosed through CT scans and DNA testing. Treatment for autosomal dominant PKD generally involves giving the patient medicine for the pain caused by the cysts, managing the high blood pressure (which can make damage to the kidneys worse), and dialysis or kidney transplants when kidney failure occurs. There is no cure.
Autosomal recessive PKD, which is sometimes called "infantile PKD", is a rare inherited form of the disease; both parents must be carriers of the disease gene for a child to be born with the condition. The symptoms are nearly identical to autosomal dominant, but affected people are born with symptoms; sometimes trouble begins when the child is still a fetus. Children with severe cases may die before or shortly after birth. Children with milder forms generally suffer from stunted growth and liver problems in addition to their kidney problems; they usually die before the age of 30. The disease can be diagnosed in unborn babies via amniocentesis or chorionic villus sampling.
A third form, acquired cystic kidney disease (ACKD), can develop in people who have scarring in their kidneys (often due to long-term dialysis). The disease is not inherited. The cysts that develop often bleed and cause excruciating pain. Most people who develop this condition are already undergoing treatment for kidney problems, though surgery is sometimes necessary.
Part of the information in this writeup was gleaned from http://www.niddk.nih.gov/health/kidney/pubs/polycyst/polycyst.htm. The rest is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/