A Secret Santa is where the participants send in their name and address to a person who acts as a coordinator. That person then pairs people off for sending gifts. Once you have joined, you receive a name and address to send a gift to. But the twist is that the person you are sending the gift to is NOT the person that will send you a gift. Hence, the Secret Santa.
After Christmas the participants will list in a thread what their Secret Santa's sent them. Optionally, the Secret Santa for that person may optionally reveal themselves if they so desire.
As you can imagine, and as I have stated above not all participants play equally. But judging from the amount of happy people after Christmas I think that most do play fair.
The last Secret Santa gift I sent never sent in an acknowledgement that it was received. Did the gift even arrive? I have no idea. In fact, as far as I know that member never came back to this website.
But the other years I participated I did receive some very nice gifts!