Breaking the Caesar cipher is trivial as it is vulnerable to most forms of attack. The system is so easily broken that it is often faster to perform a brute force attack to discover if this cipher is in use or not. An easy way for humans to decipher it is to examine the letter frequencies of the cipher text and see where they match those found in the underlying language.
By graphing the frequencies of letters in the ciphertext and those in the original language of the plaintext, a human can spot the value of the key but looking at the displacement of particular features of the graph. For example in the English language the frequencies of the letters Q,R,S,T have a particularly distinctive pattern.
Computers can also do this trivially by means of an auto-correlation function.
As the system only has 26 non-trivial keys it is easy even for a human to cycle through all the possible keys until they find one which allows the ciphertext to be converted into plaintext.
Known plaintext attackEdit
If you have a message in both ciphertext and in plaintext it is trivial to find the key by calculating the difference between them.