What is a "Signup" vs a "Paid Member"?

It appears Affiliates plugin generates a report for the site as a whole (all affiliate activity) and also for each individual affiliate. Each of these reports seems to include analytics for "clicks", "signups" and "paid members".

"Clicks" seems self-explanatory to me...it's the number of visitors brought to the site through that affiliate's referral link.

"Signups" I'm not clear on. When an affiliate sends a visitor to the site, when does the "sign up" occur? How is this defined and tracked?

"Paid members" I'm assuming is the number of completed sales generated from that affiliate's referral link. Is that accurate?

It's the "signup" I'm mostly not sure about. Is it opt-ins? Any insight is much appreciated.