From Nookipedia, the Animal Crossing wiki
Furniture in the Animal Crossing series can be used to decorate homes, thus improving Happy Room Academy scores and winning prizes, especially if placed properly. There are various themes and series of furniture ranging from Cabana to Robot, Regal to Modern, Lovely to Exotic, and much more.
Using furniture to design houses and public facilities is a major element of Animal Crossing: Happy Home Designer, a spin-off based on the main series.
Furniture can also be given to the player by villagers via letter or as an errand reward, found in the Lost Property, checking the recycle bin, by helping villagers or shaken from trees. Only two pieces of furniture can be found in trees per day. Certain special characters such as Pascal give furniture, and certain series can be obtained at Events. Also, sometimes if a villager is in a good mood, they will give the player something at random, or if they need help "recycling" one. The player can also use a slingshot to hit presents attached to floating balloons, and they will get a random piece of furniture. (Animal Crossing: Wild World, Animal Crossing: City Folk only) In Animal Crossing (GCN), the player would simply wait for the present to get stuck in a tree, then shake the tree to access it, due to the absence of the slingshot.
Furniture (and some other items, such as gyroids and fruit) is unlocked little by little as the player designs villager houses. Usually the furniture that is unlocked for each house relates to the theme the villager was asking for.
Furniture appears as a leaf when dropped outside. When placed inside a house, it appears as the full item. There are several types of furniture, but only 4 sizes:
Groups of furniture
Series consist of common, traditional real-life furniture (such as beds, tables and lamps) all with the same theme and appearance. Examples include the Blue series, Lovely series and event-exclusive furniture like the Harvest series. There are ten pieces of furniture in a series (eleven in Animal Crossing: New Leaf), plus a wall and floor. One of these pieces is usually a rare spotlight item. Happy Room Academy bonuses for complete Series are:
20,000 points for the ten (eleven in Animal Crossing: New Leaf) furniture pieces without the matching wall or floor.
25,000 points for all furniture pieces and either the matching wall or floor.
30,000 points for all furniture pieces with both the matching wall and floor.
Themes do not consist of common real-life furniture. Instead they consist of items that are all related to a specific theme, but are unconventional and not normally used in the home, and may resemble models or souvenirs. The number of items in a Theme varies between Themes. Examples of themes are the Space theme and Construction theme. Having a complete theme is worth a Happy Room Academy bonus of 3,000 points plus 3,000 points for every item in the complete Theme. There appears to be no complete Theme bonus in Animal Crossing: New Leaf.
Sets are small groups of furniture with a common theme, but do not usually have a matching wall or floor. For example, having a red sofa and red armchair will complete the red furniture set. Happy Room Academy bonuses for complete Sets are 1,000 points times the number of items in the set.
Furniture categories are larger groups of items that share a particular function and were first introduced in Animal Crossing: Wild World. They provide the player with an HRA bonus if enough items from the same category are placed in the same room. Originally there were only four categories: Plants, Musical Instruments, Paintings, and Models, until a fifth category, Dolls, was introduced in Animal Crossing: City Folk. These categories were further expanded in Animal Crossing: New Leaf where dozens of new additional items were collectively added to the categories. New Leaf also saw the renaming of the Paintings category to "Art" and the shortening of the Musical Instrument category to simply, "Instruments".
A new feature in Animal Crossing: New Leaf; once the player has sold 100,000 Bells worth of items to Reese, has 10 shirts and 50 furniture items in the catalog, and has played the game for at least 7 days, Cyrus wakes up at Re-Tail. For a small fee, he can change the appearance of certain furniture. Options include changing the color of the furniture, adding a design or using ores to create a crystallized effect. Customized furniture has a furniture leaf of a different color rather than the standard green, depending on how it was customized.
Types of furniture
Chairs & sofas
Chairs & Sofas are types of seating that also include armchairs. Players can sit on chairs and sofas.
Beds are entered by the sides and can be switched with the original beds in the attic. A player lying on a bed will have the eyes closed. If beds are lined up, a player can roll across them as if there is no gap between them. If there is one double bed with two players on it, they can lie on each side, but they will bump into each other if one tries to move to the other side while the other one is still there.
Lamps, also known as lights, are used to light up rooms. They can be turned on and off using the A button while the player is facing them. In Animal Crossing (GCN) players can light the room using the Z button. If players do not have a lamp in a room, that room will stay dark.
Storage Units in Wild World and City Folk replace the basement from Animal Crossing (GCN). These include dressers, wardrobes, cupboards, cabinets, and refrigerators. They can hold up to 3 items in Animal Crossing, 90 in Animal Crossing: Wild World, 160 in Animal Crossing: City Folk and 180 in Animal Crossing: New Leaf.
Clocks show what time it is and move as real time passes. Some clocks do different motions every hour.
Decorations include Plants, Paintings, Musical Instruments, Trophies, Mementos & Models, Vases, screens, vanities, dolls, fireplaces and other miscellany. They serve no specific purpose other than to improve the house and add to Feng Shui, if positioned correctly. They can be placed on the ground and on Tables or can be saved for later in Storage. Some items, such as the fireplace and the backyard pool, are animated.
Music players, or Stereos, are used to play songs that K.K. Slider gives each Saturday night. Songs are set on a loop and may sound quite different from K.K's original song, but still retain the basic tune. Only one song can be played in each room at a time.
New in Animal Crossing: New Leaf, this type of furniture is placed on a wall rather than the floor. Examples of wall furniture include clocks, fans and lamps. Wall furniture is sold at Super T&T, T.I.Y. and T&T Emporium.
Fish and bugs can be caught outside and placed inside a house. Fish will appear in an aquarium (the size changes depending of the size of the fish) and bugs will appear in a glass box, making noise if they can.