Firstly, all questions need a question mark "?" so don't forget those. Anyway, the difference between the two is 'have' vs. 'had', so it's past versus present. "What have you been doing?" is present perfect continuous. It's asking what you've been doing up until now, or in the recent past. "What had you been doing?" is past perfect continuous. It's asking what you've been doing up until, or in the recent past of, a point in the past. You'd either need to make that point in the past part of the question, or it would need to be implied by the conversation so far. So think of it as: "What have you been doing?" = "What have you been doing until now?" or "What have you been doing recently?" "What had you been doing?" = "What had you been doing when that happened?" or "What had you been doing until then?"