Pen pals (or penpals or pen friends) are people who regularly write to each other, particularly via postal mail.
笔友就是对那些定期通过邮政信件彼此联络的人们的称呼。
A penpal relationship is often used to practice reading and writing in a foreign language, to improve literacy, to learn more about other countries and life-styles, and to make friendships.
笔友多用来提高外语阅读和写作能力、增长知识、了解其他国家和不同的生活方式，以及用来交朋友。
A modern variation on the traditional penpal arrangement is to have a keypal. People can use the keyboard to talk to each other through emails or instant messengers instead of writing 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. But people may rely too much on instant communication and lose their real-life communication skills.
传统笔友的现代版本就是交一个“键友”或“网友”。人们可以敲击键盘通过电子邮件或即时通讯工具跟彼此交流，不用再写纸质信件了。这样做的好处是省钱、即时，在短时间内就可以互通很多信息。不过，人们也可能因为过度依赖即时交流方式而逐渐丧失在现实世界的沟通技能。