From Nookipedia, the Animal Crossing wiki.
Tom Nook (たぬきち, Tanukichi) owns the town shop. After the player buys or sells certain amounts of money in his shop he will expand his shop, after which a larger range of items will be available to the player each day. After the final expansion, Nookington's department store, if playing Wild World, he will employ Harriet, who runs the hair salon, Shampoodle, and Timmy and Tommy, his nephews, in every Animal Crossing series game, excluding Animal Crossing: New Leaf, who work on the second floor. His birthday is believed to be on July 28th. Although he appears to be a raccoon, he is actually a tanuki, a Japanese raccoon dog that is commonly mistaken or translated to raccoon.
After the player moves in, Tom Nook takes the player under his wing and has the player work at his shop to pay off their debt.
Tom is very money-driven, and many years ago left behind his friends, most importantly Sable, to travel to the big city. Whole there, he has dreams of success which are quickly broken by the harsh life there. After making nothing of himself, he returns to the player's town to set up his store. Nook no longer appears to be friends with Sable, though the two often mention each other to the player.
Nook seems to have a varied reputation amongst the villagers, with some speculating that he is, in fact, a man in a suit. His large, bushy tail seems popular amongst lazy people. Many will say that he has a checkered past, hinting at illicit activities. He is openly derided by Redd as a "raccoon in an apron." His picture quote is "I'll be there with bells on! Ho ho!"
Tom Nook's description in the City Folk's game guide is:
A kind raccoon with a heart of gold, Tom Nook helps you get started when you first move to town by loaning you a home to live in. Tom also runs a little supply shop in town that sells all the basics, including furniture, tools, and gardening supplies. Tom will help you in a variety of ways as your life unfolds, and you'll help him as well: keep shopping at the store and it'll get bigger and bigger. Eventually, old Tom will even need to hire his two nephews, Timmy and Tommy, just to keep his business running!
At the beginning of the game Tom Nook's shop is just a small wooden shack like building. But after the Player buys enough from him, the shop will expand to a 2-story supermarket. There are a total of four expansions for his shop like this:
After a period of time with Nookingtons, Tom Nook will ask the player a question to "better serve his customers". Based on the answer given, Nookington's can close and reopen as a previous store to better suit the player's needs. Periodically, after the first questioning, Tom Nook will ask again, and the player can change their store again. This event only happens in City Folk. The suggestions and answers are:
 Other appearances
Tom Nook appears as a trophy in Super Smash Bros. Melee.
Tom Nook appeared in Dōbutsu no Mori (film) as the town shop owner, just as he does in the games.
Sometimes, upon entering his store, he will appear in thought, and may tell the player a story when approached: