The Comprehensive USMLE Guide For Pakistani Medical Students (Part 3: Types of USCE)

USCE: United States Clinical Experience, it's a general term that covers any clinical experience in USA.

Externship is an organized training program designed by hospitals and medical institutes to train "externs" by familiarizing them with the US medical care system without allowing them to interfere with patient management.

You might be allowed to take patient history and perhaps examine patients and write patient notes under guidance.

The program usually follows certain rules likes timing, schedules, assignments, insurance liability requirements, etc.

Most externship programs do not pay externs and many of them do require fees to be paid by the trainee in order to enroll in the externship program for certain duration. The term hands-on clinical experience usually refers to externship. Because externship is thought to be more effective than observership in getting students and grads into the US health care system, many commercial companies took advantage of this and started offering Paid Externships for IMGs.

Observership and as the name implies is about letting you observe what's going on, shadowing the professors and doctors in hospital rounds or in outpatient clinics. Observership is usually less organized and tend to have no rules. They are managed by individual physicians and  this is commonly seen in private practice. In observership usually you are not allowed to take history or examine or write patient notes [the basic rules is Don't Touch].

Sometimes, medical institutes and Universities cannot offer (externships) to IMGs due to State and local laws and regulations. Such hospitals usually provide observership rather than externship training. For example, Cleveland Clinic Observership Programs. However you can still do observerships even after you’ve graduated. So this is a viable course of action for attaining USCE + LORs for IMGs who plan to give USMLE after their house-job or graduation.

Electives is a term used for an academic course chosen by the student from a set of options, as opposed to a required course (called Core Clinical Rotation or Clerkship) which are deemed essential for an academic degree.
Elective rotations tend to be more specialized and usually have fewer students than the usual core rotations. International Medical Students can do electives outside their country, for example in the United States. This can expose them to US healthcare system early on and increase their chances of getting into residency. IMG graduates are not allowed to go into elective rotations once they graduate.

Internship is a period of medical training (usually one year) that is a mandatory requirement before a license to practice medicine is granted. Internship timing and duration differ significantly from country to another. In USA, medical students enter their internship when they start residency, that's why first year (PGY1) residents are called interns. In other countries, internship might be a period of clinical clerkship that must be attended before the student is granted a graduate status.

What about Research?
Doing research in academic institutions is of great value for both IMGs and AMGs alike. It's considered as USCE whether it's basic (lab) research or clinical research. Many Universities and research institutes offer a range of positions such as research observerships, electives, and externships. In addition, there are paid research jobs, such as research associate and research assistant, these paid jobs do not require the IMGs to be licensed in USA.

Ranking (Which is better Observerships or Externships or Electives or Research?)
If we want to rank them from the best to the least effective in getting you into residency then I'd probably rank them this way; Electives > Research > Externship > Observership
But that's relative, because probably a 3 weeks observership in a prestigious University is better than 6 weeks electives in non-ACGME accredited community hospital.

It's generally perceived that student electives entails the best possible USCE that IMGs can get, since they are organized under a med school faculty and geared toward training. Electives are even superior to Externships in that hospitals allow the elective trainee to take histories, do physicals, write notes (type EMR), present cases, ...etc. There are no States in USA that ban elective international medical students from performing these activities, while as I mentioned above, there might be such restriction with regard to externships.