Caesar and ROT13
Shift every letter a fixed number of places along the alphabet. Julius Caesar used a shift of 3. ROT13 uses 13, so running it twice gives back the original.
Result
Crack it
A Caesar cipher has only 25 possible keys, so you can simply try them all. Each line below decrypts the text above with a different key, sorted by how closely its letter frequencies match English. Choose a line to use that key.
How it works
Suetonius wrote that Caesar replaced each letter with the one three places further down the alphabet, so A became D and B became E. Letters near the end wrap around to the start. To decrypt, shift back by the same amount.
ROT13 became a Usenet convention for hiding spoilers and punchlines. Nobody thought it was secure. It just stopped you reading the answer by accident. Because 13 is half of 26, the same operation both encrypts and decrypts.
Frequency analysis breaks every simple substitution cipher. In English, E, T, and A are the most common letters. Shift a ciphertext until its most common letters line up with those, and the message usually appears. The cracker above scores each key with a chi-squared test against typical English letter frequencies. Short messages don't give it much to go on, so check the next few lines too.