Munchausen Sydrome is a disorder directed at the self. The person will invent illnesses or self-induce them in order to obtain treament and attention.

Munchausen Syndrome by Proxy is a disorder where a parent, usually the mother, will subject her child to medical procedures to cure a variety of imagined ailments. This disorder has been written about extensively, but is not a recognized disorder in the DSM-III.

Munchausen's Syndrome (sometimes Munchausen Syndrome), pronounced mun/chau/zen.

A factitious disorder named after the baron of Munchausen who was an 18th century German soldier. A book in his name, Baron Munchausen's Narrative of His Marvellous Travels and Campaigns in Russia, was revealed some 40 years later to have been written by someone else, and his name came to be synonomous with telling a lie and getting away with it.

A Munchausen's sufferer will often have a very detailed knowledge of medicine, particularly diseases and surgical procedures. S/he will often 'pick' a disease or physical disorder from a medical textbook and exhibit the symptoms of that disorder. Subjective and hard-to-confirm symptoms like pain are most common. Typically, the patient will undergo several rounds of exploratory surgery and often quite invasive procedures in an attempt for a cure. Some patients will sabotage their treatment in an attempt to cause complications, others will be "cured" until they start to feign symptoms of another illness. Some sufferers actually go to lengths to contract a genuine disease or injure themselves so they can be treated.

Munchausen's sufferers requires psychiatric help and is known to be very difficult to cure.

Not to be confused with Munchausen by Proxy Syndrome, where the sufferer harms another person such as a child.

sources:
http://my.webmd.com/content/c4_asset/merriam-webster_medical_dictionary_165330
http://www.merck.com/pubs/mmanual/section15/chapter185/185d.htm