Training + patients. They complete different residency training programs after medical school graduation. Internists specialize in medical (not surgical) care for adults, whereas family physicians care for adults, children, and may also do some obstetrics or surgery. There is more overlap with primary care internists, who may also do women's health care (but don't deliver babies or see children).
Few. Most fm work in outpatient settings where many im work as hospitalists as well as outpatient fm see patients of all ages as well as ob. Im usually only see adults.
Patient traits. In addition to the previous answer, it is important to recognize that internists specialize in following patients with multiple, chronic diseases; such as would be seen in the older, adult population. Internists have also become the primary hospitalists of today, taking care of adult patients when they are admitted.