Oh, it's fun! Here's the rules, from Wikipedia:
Rules
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1. All gifts should be valued no more than $15. The gift also can be a old trinket, knick-knack, or unwanted gift from previous years looking to be 'regifted'. (Ex: gag gifts, alcohol,gift cards, home made gifts) Use your imagination and let's make this fun.
2. Participants bring their gifts wrapped or sealed in a gift bag, and often take care to hide the contents of the gift and the identity of the giver.
3. Gifts are placed in a designated area. (Place your wrapped gift on the table of the main conference room).
4. Participants draw numbers to see who goes first, second, etc.
[edit] How to Play
1. The guest who has drawn #1 goes first. He/she chooses a gift from the gift pile, opens the gift for all to see, and keeps it for the time being.
2. The guest who has drawn #2 goes next. He/she can either choose to take an unopened gift from the gift pile or to "steal" a gift from any previous gift-opener.
3. If a gift is "stolen" from a previous gift-opener, he or she may either open an unopened present from the gift pile, or "steal" another guest's opened gift. Turns resume when all guests who have taken their turn possess an open gift.
4. After all turns have been taken, the person who has drawn #1 has the option of taking one more turn (stealing any gift) since they had the disadvantage of not knowing what any of the now-opened presents were in the beginning.
OPTIONAL RULE: A gift is considered "FROZEN" once it has been "stolen" four times, and, once frozen, it becomes unavailable to the other guests---it then belongs to whoever possesses it at that time, and cannot be stolen again. If someone has donated a gift that stands out among the rest, this prevents the game from revolving entirely around that gift.
Origin
The name "Dirty Santa or White elephant" loosely refers to an old tradition of giving second-hand gift items that are either kitschy or burdensome for the owner to maintain. It originates from an old Burmese cultural reference to the value of an albino elephant, which is considered sacred and must be well taken care of, despite the financial burden that it may bring to its owner.
It can get pretty cutthroat if there's a gift everyone wants!
/emoticons/
