Caesar shift
Every letter moves forward in the alphabet by the shift amount, wrapping from Z back to A. A shift of 3 turns HELLO into KHOOR. Numbers, spaces, and punctuation pass through unchanged.
Encode and decode
Decoding reverses the shift, so decoding with the same shift value used to encode returns the original text. The one exception is a shift of 13: since 13 is half of 26, encoding and decoding produce identical output, which is why ROT13 has no separate decode mode.
ROT47
ROT47 extends the same idea across all 94 printable ASCII characters instead of only the 52 letters. Hello, World! becomes w6==@[ (@C=5P, with the digits and punctuation rotated along with the letters. Like ROT13, ROT47 is self-inverse because 47 is half of 94.
Where these ciphers are used
Caesar shifts show up in puzzle hunts, cryptography exercises, and escape rooms. ROT13 and ROT47 turned up historically in Usenet posts and forum threads to hide spoilers or profanity from a casual glance, not to provide real security.