What is the difference between has seen and had seen?

has seen - is present perfect (third person) - he/she/ the doctor has seen

had seen - is past perfect (all persons) - he/ she / the doctor / we / they had seen.

has seen - present perfect is used to talk about something that happened in the past and is related some way to now.

eg He has seen the movie before.

had seen - past perfect is used to talk about something that happened in the past before another thing (usually past simple) that happened in the past.

eg He had seen the movie when he went to Hamilton.