Until recently, letters were an extremely common way for people to communicate over long distances. People would hand-write their thoughts on paper, seal the letters in an envelope, and pay a few cents to send those thoughts anywhere in the country. For a few cents more, they could send their ideas anywhere in the world.
In terms of cost, letters are still the most efficient way for people to send their ideas a long distance.
Letters are also much more personal than communicating electronically. They provide the recipient with a lasting reminder of the sender's thoughts, in the sender's handwriting.
In fact, historians are worried that much of history is going to now go unrecorded, since great sections of history have been written based on the letters sent and received by important historical figures. Electronic communications may be recorded someplace, but they're often difficult to track down, and even harder to access.
There are two main types of letters: friendly (or personal) letters and business letters.
Click here to view some examples of letters.