How it works
With a shift of 3, A becomes D, B becomes E and Z wraps around to C. “Hello, World!” becomes “Khoor, Zruog!”. Decoding means shifting back by the same amount. Only letters change; digits, spaces and punctuation stay the same.
Where the name comes from
The Roman writer Suetonius reports that Julius Caesar wrote private letters by moving each letter three places along. The idea is simple enough that variants have turned up ever since, including ROT13, a shift of 13 that undoes itself when applied twice.
Why it is easy to break
There are only 25 useful shifts, so trying each one takes moments. It also preserves letter frequencies: the most common letter in the coded text probably stands in for “e” in English. That is enough to recover the shift without a computer.
Where it is still useful
Puzzles, teaching and hiding a spoiler. It is never suitable for anything private; for that, use encrypted tools. Try it in the Caesar Cipher tool or read how to use a Caesar cipher and ROT13.
Source: Suetonius, Life of Julius Caesar, section 56 (Caesar’s use of a three-place shift).