Internal medicine is the medical specialty that deals with the non-surgical treatment of illness in adults. A specialist in the field is known as an internist (as opposed to an intern, who is a medical trainee). Internists often deal with patients who have diseases that affect multiple systems of the body or have an undifferentiated illness. Many other specialties in medicine are, in fact, sub-specialties of internal medicine including cardiology, nephrology, endocrinology and infectious disease. An internist will have a very broad range of knowledge, but it will not be very "deep". Although they often treat patients on an out-patient basis, many of their patients are hospital bound and very ill. In most cases, an internist will be the primary attending physician for a patient and will oversee the other sub-specialists working on the case.
An internist will not work on pediatric patients, as the presentation and treatment of disease in adults usually differs greatly from that in a child. An internist will also generally not work on patients who clearly require surgery, or patients who are pregnant.
Related source: Also check out internal medicinefor updated information on internal practice and specialization.