Present Perfect Tense is used to express an action that began in the past and continues in the present or its effects are still there in the present.
For example: Who is this boy? I have never seen him.
The room is dirty. Nobody has cleaned for a long time.
On the other hand, Past Perfect Tense is used to express an action that started in the past and completed in the past time itself. It has no effects in the present time.
For example: I didn’t know who he was. I had never seen him.
The room was dirty. Nobody had cleaned it for a long time.