Internal medicine is the branch and specialty of medicine concerning the diagnosis and nonsurgical treatment of diseases in adults, especially of internal organs.

Doctors of internal medicine, also called "internists", are required to have included in their medical schooling and postgraduate training at least three years dedicated to learning how to prevent, diagnose, and treat diseases that affect adults.

Caring for the whole patient 

Internists are trained to solve puzzling diagnostic problems and handle severe chronic illnesses and situations where several different illnesses may strike at the same time.

Subspecialties of internal medicine 

Internists can choose to focus their practice on general internal medicine, or may take additional training to "subspecialize" in one of 13 areas of internal medicine, generally organized by organ system. Cardiologists, for example, are doctors of internal medicine who subspecialize in diseases of the heart. The training an internist receives to subspecialize in a particular medical area is both broad and deep. Subspecialty training (often called a "fellowship") usually requires an additional one to three years beyond the standard three year general internal medicine residency. (Residencies come after a student has graduated from medical school.)

In the United States, there are two organizations responsible for certification of subspecialists within the field, the American Board of Internal Medicine, and the American Osteopathic Board of Internal Medicine. 

The following are the subspecialties recognized by the American Board of Internal Medicine.

Cardiology, dealing with disorders of the heart and blood vessels 
Endocrinology, dealing with disorders of the endocrine system and its specific secretions called hormones
Gastroenterology, concerned with the field of digestive diseases
Hematology, concerned with blood, the blood-forming organs and its disorders
Infectious disease, concerned with disease caused by a biological agent such as by a virus, bacterium or parasite
Medical oncology, dealing with the study and treatment of cancer
Nephrology,dealing with the study of the function and diseases of the kidney
Pulmonology, dealing with diseases of the lungs and the respiratory tract
Rheumatology, devoted to the medical diagnosis and therapy of rheumatic diseases.

The ABIM also recognizes additional qualifications in the following areas
Adolescent medicine
Clinical cardiac electrophysiology
Critical care medicine
Geriatric medicine
Interventional cardiology
Sports medicine
Transplant hepatology

Internists may also specialize in allergy and immunology. The American Board of Allergy, Asthma, and Immunology is a conjoint board between internal medicine and pediatrics. 

The American College of Osteopathic Internists recognizes the following subspecialties.

Allergy/Immunology
Cardiology
Critical care medicine
Endocrinology
Gastroenterology
Geriatric medicine
Hematology/Oncology
Infectious diseases
Nuclear medicine
Nephrology
Pulmonology
Rheumatology

References

Other websites 
cell research - the world's sickle cell research center,burla,orissa,India
American College of Physicians - the world's largest medical specialty society for doctors of internal medicine
Doctors for Adults.com  - The American College of Physicians' patient education Web site, with more info about internal medicine and internists
On-line Internal Medicine Journal Club (via JournalReview.org) 
The American Board of Internal Medicine - The largest certifying board for internists and related subspecialists in the USA
Canadian Society of Internal Medicine 
The American College of Osteopathic Internists
The American College of Osteopathic Internists
Internal Medicine positions  - Internal medicine positions available
 Internal Medicine- CME Internal Medicine Org

Medical specialties