Clerkships or electives refer to the programs that are taken by medical students in their 3rd or 4th year at the hospital of medical schools or its affiliates. This includes an approval process and is also based on your USMLE scores.
In Observerships, you are able to shadow preceptors and there would be no direct contact with patients. You can actively participate in case presentations and earn recommendations.
Externships are open to IMGs, which gives hands-on clinical experience. This can be beneficial for the residency match process. You can select your specialty and gain Letters of Recommendation based on your performance.