6 Answers

A pen pal is a person in another city or country (Somewhere that isn't easy to visit)... that you write letters to and they write back.  Seems quite old fashioned now but it was fun when I was a child.

It was very exciting when a new letter arrived.

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

Now days pen pals can be via emails. It's writing back and forth to an otherwise stranger and communicating with them via writing......

Pen Pal is kinda like a facebook friend but it was done with this device that went between your fingers and you would scribble on paper then lick this sticker thing and stick it in a envelope and put it in a blue metal box.  So complicated

...................and it is generally someone you never get to spend face-to-face, one-on-one time with, hence the term "pen" pals (pen being the writing instrument).  
Then again, there are "pen pals" in prisons, but we don't want to go there.  

Someone who gives you his "Pen" when you haven"t got one.