I think internal medicine (usually just called medicine) is the comphrensive, scientific study of human disease. There are two main types of graduate study in internal medicine: general im, which is designed for people wishing to pursue primary care, and categorical, which is designed to serve as a foundation for further subspecialization (ie oncology, hematology, gastro, cardio, etc).
Family practice is designed to prepare a physician for only primary care, usually private practice. I think it emphasizes personal relationships, patient satisfaction, etc. I don't know that it is the most competitive field of medicine.
OB/GYN and pediatrics are also primary care fields. It is important to note that the number of general IM slots and family practice slots left empty after match-day has been steadily increasing in the past decades. There are many reasons for this...