Pen pals (or penpals or pen friends) are people who regularly write to each other, particularly via postal mail.

Purposes
A penpal relationship is often used to practice writing and reading in a foreign language, improving literacy, to learn more about other countries and life-styles, and to ameliorate loneliness. As with any friendships in life, some people remain penpals for only a short time, while others continue to exchange letters and presents life-long. Some penpals eventually arrange to meet face to face.
Penpals come in all ages, nationalities and cultures. Pals may seek new penfriends based on their own age group, a specific occupation, hobby, or select someone totally different from them to gain knowledge about the world around them. Being part of a penpal network can be a way to gain a valuable understanding of the world, and an appreciation for cultures and lifestyles very different to your own.
Many people writing to a penpal are not using their native language in their letters.
A modern variation on the traditional penpal arrangement is to have a keypal and exchange email addresses as well as or instead of paper letters. This has the advantage of saving money and being more immediate, allowing many messages to be exchanged in a short period of time. The disadvantage is that the communication can be very ephemeral if the email messages are not routinely saved. Many people prefer to receive paper letters, gaining the satisfaction of seeing their name carefully printed on a thick envelope in the letterbox. Using postal mail, it is possible to trade postcards, stamps and anything else light and flat enough to fit inside an envelope, often called "tuck-ins".
Penpal clubs can be found on the Internet, in magazine columns, newspapers, and sometimes through clubs or special interest groups. Some people are looking for romantic interests, while others just want to find friends. It seems, on the internet, that the term "pen pals" defines those looking for relationships, where pen pals originated via postal mail correspondences and has evolved to mean something more. Penpals also make and pass around friendship books, slams and crams.
In recent years, penpal correspondence with prison inmates has gained acceptance on the Internet. Most pen pal newsletters do not publish prison pen pal ads because these communications require more caution, not simple friendships. Prison penpal sites are proactive, though, in offering full disclosure of inmates' crimes and providing advice for would be pen pals.