From Nookipedia, the Animal Crossing wiki
An Igloo is the Winter equivalent to the Summer Tent. They are set up in January, at the same time as when snowballs start appearing in the town. One of the villagers in the player's town will set up an Igloo on one of the Signposts. Inside, one villager is set up with a large pot of chowder over a warm fire and a rug (in Doubutsu no Mori+, in place of the chowder was a small wok with slowly-bubbling blocks of tofu). The player can either just chat to the villager in the igloo, or can play a game with them to win exclusive items. (Note: These items were only exclusive in Doubutsu no Mori+, Animal Crossing and Doubutsu no Mori e+. They could be bought repeatedly from Tom Nook in Animal Crossing: Wild World and Animal Crossing: City Folk). Games are unique to each personality.
Please note that the DUMMY can only be obtained in the US version of Animal Crossing and cannot be obtained in any other version of Animal Crossing, including Doubutsu no Mori, Doubutsu no Mori+ and Doubutsu no Mori e+. And also, the character's personality will effect the choice of prizes. For example, if one plays with Axel, he or she may never get the rare item DUMMY, but if playing with somebody like Purrl, there is a small chance of getting it.
Igloos return in Animal Crossing: New Leaf. But this time, villagers that are not from the player's town will set up their Igloo on the Campsite in the player's town, which functions identically as the Tent Camper. The camper can also ask to move into the player's town. If the player agrees for the camper to move in, an area in the player's town will be reserved by Nook's Homes, but only if the town has 9 or less villagers in it. Likewise, if the player disagrees, or if the town has 10 villagers in it, the camper will not move into the player's town. If the player's pockets are not full, the camper may either play a game in where the player buys or sells an item. Campers will also ask to buy shovels, axes, watering cans, fishing rods, nets, slingshots, timers, megaphones and any carpet, wallpaper or furniture that the player has had for a while. However, when playing a game, the player can obtain Igloo Camper items, all that cannot be found in Timmy and Tommy's store, but can be re-ordered from the catalog.