Internship: first year of residency
Residency: training leading to Board Eligibility in a specialty
Fellowship: advanced training leading to super specialty (not required)
Attending: someone who has completed their training
Technically, it is NOT required to finish residency in the US to practice, as long as you complete the state requirements for a medical license. However, completing a residency is often required by malpractice providers, insurance companies and hospitals/employers so failure to complete a residency significantly limits one's practice choices.