Hi Virginie,
First thing is that this should be in the Adobe Campaign Classic forum as the two products are distinct enough that you'll likely get your answers faster if they are in the right forum.
For the indicators in question the big distinction is people versus recipients.
When Adobe talks about recipients it means known recipients that exist in the Campaign database and in this case, were part of the original delivery target. People on the other hand, consist of and person known or not to Campaign that may or may not have been in the original target of the delivery. So in short, when a tracking indicator such as Distinct clicks for population reached mentions "people" it's basically saying anyone including people who had the delivery forwarded to them are being counted. When Recipient clicks is used its only counting whatever you defined in your workflow or recipient list that was fed into your delivery.
Hope that helps.
Craig
