A Substitution Cipher is similar to a Caesar cipher, but instead of using a constant shift left or right, the plain alphabets and the cipher alphabets are mixed arbitrarily.
Plain Alphabet: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Cipher Alphabet: Z Y X W V U T S R Q P O N M L K J I H G F E D C B A
With the above, the Plain text "This is a sample" would encrypt to "Gsrh rh z hznkov."
With Substitution Ciphers, the secret is in the mapping between the plain and cipher alphabets. However, there are several analytical techniques to help break these ciphers with only the ciphertext. See Frequency analysis