Internists generally get trained pretty heavily in hospital work. They tend to be more detail oriented, leave less rocks unturned in working you up, and order more tests. They really know their stuff well. They only treat adults.

Family Practice training is often more outpatient based, more broad, includes both pediatrics and OB/GYN, can be more procedural (FPs are often trained to do C sections, vasectomies, etc), often get more musculoskeletal training (casting, setting bones, etc). Their depth of knowledge tends to be not to the level of the internists.

General Practitioners are a breed that is slowly dying out, except in rare circumstances. They are people who did not complete a whole residency training. They usually graduated from medical school and only did one year of training afterwards then jumped into practice (vs Internal medicine and FP who do 3). Often these are the older guys. They were much more common 40 years ago, as a lot of doctors got their first training working in the wars, then would go to medical school and really not need a residency as much. Also, in the 50s and 60s the amount of medical knowledge needed to be a doctor was very small compared to what it is today, so the need for a residency was not nearly what it is today.

These are very broad generalizations and of course there is plenty of overlap between the fields and plenty of exceptions to these stereotypes. If I were you I would look at your needs. If you have some more complicated medical problems and just want yourself treated, generally an internist would do a good job. If you don't have many issues or have a desire for your doctor to be your wife's doctor and your kids' doctor, go with FP. Overall though, just ask around and choose the best out there notwithstanding the specialty from what others tell you. Much more important than the training they received is how good they are to their patients.

This is coming from somebody who is going to be starting a family practice residency here in 8 months.