Rot13

Rot13 rotates all 26 letters of the alphabet by 13 positions rolling over to position 1 after position 26. A sample display of how rot13 "encrypts" is shown below:

$ echo A B C D E F G H I J K L M N O P Q R S T U V W X Y Z | rot13
       N O P Q R S T U V W X Y Z A B C D E F G H I J K L M

So you see A becomes N and N becomes A. This encryption can be found in many places online, such as Usenet and geocaching.com.