There are some differences between the two professions, but most of them are minor, and the only significant difference is that an internal medicine doctor has chosen to specialize in treating adults and seniors, but not children. A general practitioner will treat children too. Children have particular needs just as adults have particular needs, so a physician may choose to specialize in meeting the needs of one age group, which is what an internal medicine doctor does. After an internal medicine doctor completes an internal medicine residency, he or she is known professionally as an internist.