Physician is anyone that has gone through medical school and passed their Boards. This follows them getting a Bachelor's degree and then attending a medical school and then passing Boards.
Medical students are still attending medical school and technically not a "doctor" yet - they may write "orders" in places but they cannot be initiated until signed off by a doctor.
A SubI (sub intern) is a MD that has not started official internship - usually seen in the late spring and summer between graduating med school and starting internship.
An intern is someone that has completed med school, passed boards - but is only in their first year of being an MD. Virtually all MDs must do some kind of internship, and residency before being allowed to practice independantly...something that should be required for nurses, in my opinion, at least for a year or two post graduation. They can write and initiate orders, other than those barred by facility protocol (such as chemo or research meds)
A resident is an intern past the one year mark....most are required at minimum for at least 3-4 years, I believe, and more for many specialties.
A "fellow" has finished residency and is specializing in more advanced aspects of practice. In BMT patients, my speciality, they and the attendings write all chemo orders or sign off on them, for safety issues.
And attending is the MD that oversees them.
House staff/officers generally refers to the MD/DOss in attendance (generally interns/residents/fellows at any given time ) sometimes refers attending but not always. The term comes from earlier eras, where residents and interns actually "lived" at the hospital...either because they were on duty >80 hours a week, or lived in adjoining housing. After the 80 hour rule (interns/residents cannot do over 80 hours officially per week - though they often do more work "unofficially"), they no longer spend all that time there.
All that are interns or higher are doctors, as are DOs.