In Animal Crossing: Wild World, you can buy stationery from Tom Nook's store and use it to write letters to your villagers. When writing a letter, you have four lines of space and can use a keyboard to type up a message. You can also customize the salutation (like "dear villager") and signature parts of the letter.
If you write a letter that your villager thinks is bad or confusing, the villager will send you a bad reply. But if you write a letter that your villager likes, you will get a favorable response. Best of all, if you attach a gift to a good letter and send it to a villager, you will increase your friendship with that villager and get a gift from that villager in return. Below, I explain what makes a villager consider a letter to be good, and I will describe what sorts of gifts you can get from villager letters.
Villagers like letters that have real words in them. Villagers can recognize real words from gibberish. However, they cannot tell if you are just repeating the same word over and over again. Villagers do not really count short words like "a" and "the", but you can repeat a real word over and over again, up to eight times, and the villager will consider it to be a good letter. But if you exceed eight repetitions, the villager will consider it to be a bad letter.
Villagers don't know word meanings. Even if you put negative words in the letter or say mean things, the villager will think it is a good letter. However, I have read that villagers will consider the letter to be bad if you put swear words in it.
If you include a gift in a well-written letter to your villager, you will get a gift in response. The type of gift that you get depends on the length of the letter.
No; you can attach anything at all to the letter and, as long as your letter was good, the villager will send you a favorable reply. This means that you can send trash, forged paintings, pitfall seeds, and other things that get you little or no money from Tom Nook. This can be a good way to get value out of an item that is otherwise not worth very much. But be careful, because if you send something really valuable, you might regret it if you don't get something quite as valuable in return. Imagine sending a throne, worth 200,000 Bells, to a villager, and getting only a piece of fruit in return! Sometimes you might feel like being that generous, but if you're still paying off your mortgage, you might not want to give away something that valuable.