Family practice MD deals with everything from pediatrics to geriatrics and everything from pregnancy and cancer to broken bones and cut fingers. They're called GPs or general practicioners. They tend to make a lot of referrals to other specialists after making an initial diagnosis. The GP is likely to set a broken wrist or stitch up a laceration.
The internist, on the other hand, is more of a specialist, limiting practice to the major circulatory, pulmonary, neurological, and digestive systems - that means heart and blood system, lungs, nerves, and stomach and intestines. He tends to stay away from stuff like cuts and bruises, broken bones, etc. The internist is likely to work with ulcers, pinched nerves, and polyps.