Generally in a pyramid scheme the participants are aware that they are being paid a percentage of the funds that they recruit.  Additionally, only those who successfully recruit investors "earn" any funds.

With a Ponzi, the participants are typically mislead to believe that they are "investing" in a venture that is earning profits for all involved.  The early investors spread word of the opportunity on the belief that it is legitimate and successful.  Someone operating a Ponzi will typically try to discourage anyone from withdrawing funds, and will encourage them to "re-invest" any reported earnings.  When a few people attempt to withdraw small amounts, the Ponzi operator will pay out these amounts from the total funds collected, regardless of who they came from or whether the person withdrawing recruited anyone else.  These early small payouts give an impression that the investment is safe.  Eventually more people will want to withdraw funds than the Ponzi operator has available.  At that time, he must shut it down and it slowly becomes clear to everyone involved that they will not be getting any further funds back.  It is also possible for the Ponzi operator to run off with the funds once they have collected a large amount even if nobody is requesting to withdraw their funds yet.