I was trained as a Family Practitioner. When I completed my residency, I was setting fractures, doing surgery, delivering babies, taking care of newborns, taking care of children and adults--frankly, I was doing about everything.
Now a general practitioner did the same thing, at one time--but as specialists came into the picture, they were increasingly not allowed to do as much of the specialty stuff. These are the folks that don't complete a specialty residency (And Family Practice is considered a specialty residency, by the way...) but are still licensed to practice medicine.
Now, an internal medicine doctor, which I'd call a General Internist or an Internist, would be a physician who specialized in the medical care of adults. No surgery, and technically no pediatrics, no obstetrics, no setting bones, etc. Just handling things like hypertension, diabetes, renal insufficiency, seizure disorders, etc, etc, etc.
As it turns out, I've given up obstetrics, surgery, all but the most elementary orthopedics, etc; I do do more pediatrics than a general internist, and I still stitch up gashes and such which most general internists wouldn't do, but I'm awfully close to being one, anymore. Why? With the cost of the malpractice for a lot of the things I used to do, I can't afford it. Folks that do obstetrics pay more a year for malpractice than I MAKE a year. I'm nuts, but not that nuts!!!
OK, a really rambling explanation, but it's the straight answer from an insider...
Career in medicine, rubbing elbows with GP's, FP's and Internists, to say nothing of a ton of other specialty folks...