The Caesar cipher is a substitution cipher where each letter in the plaintext is shifted a certain number of positions down the alphabet. This shift value is often referred to as the "key" or "shift."
For example, with a shift of 1, "A" would be encrypted as "B," "B" as "C," and so on. When reaching the end of the alphabet, the sequence wraps around, so "Z" becomes "A." The same shift is applied to decrypt the message.
The Caesar cipher is named after Julius Caesar, who reportedly used it to communicate confidential messages during his time.