ROT-13 is the encrypting of a message by exchanging each of the letters on the first half of the alphabet with the corresponding letter in the second half of the alphabet (that is, swapping positions by 13 characters). Thus, A becomes N, B becomes O, and so forth, and conversely, N becomes A, O becomes B, and so forth. Numbers, spaces and punctuation are not changed. ROT-13 is sometimes used to encrypt messages that may be offensive or of questionable taste, or messages that contain spoilers (like movie endings or punch lines). The purpose of the code is not to guarantee security, but simply to make it difficult for anyone to read.
ROT-13 is sometimes used on the Internet to encrypt e-mail addresses to discourage spamming. Some browsers or Word editors offer users the ability to convert back and forth between regular text and ROT-13. Typically, you highlight text you want to encrypt or decrypt and then select the ROT-13 encode/decode button.
Here's an example of some regular text:
This is a sample of a message encoded using ROT-13 encoding. Because of the simple nature of the encryption, its purpose is not security but to prevent accidental reading.
that would look like this in ROT-13:
Guvf vf n fnzcyr bs n zrffntr rapbqrq hfvat EBG-13 rapbqvat. Orpnhfr bs gur fvzcyr angher bs gur rapelcgvba, vgf checbfr vf abg frphevgl ohg gb cerirag nppvqragny ernqvat.
ROT-13 is sometimes known as Caesar's code because the Romans General is said to have used it during the Pelloponesian Wars.