The Caesar cipher is one of the oldest known encryption techniques, named for Julius Caesar's reported use of it in private correspondence. Every letter in the message is shifted a fixed number of places through the alphabet, a shift of 3 turns A into D, B into E, and so on, wrapping back to the start after Z. It's not secure by modern standards since there are only 25 possible shifts to try, but it remains a great teaching tool for understanding substitution ciphers and is still used in puzzles like ROT13 (a Caesar shift of 13).
It's how many positions each letter moves through the alphabet. A shift of 1 turns A into B; a shift of 13 (ROT13) turns A into N. The same shift value used to encrypt must be used to decrypt.
No, only letters A-Z and a-z are shifted. Digits, spaces, and punctuation are left exactly as they are, and letter case is preserved.
Because there are only 25 possible shifts, anyone can crack it instantly just by trying each one until the text makes sense, this is called a brute-force attack, and it's why the Caesar cipher is used for puzzles and education rather than real security.