Data Compression : Finding Redundancy

Most types of computer files are fairly redundant -- they have the same information listed over and over again.

file-compression programs list information once and then refer back to it whenever it appears in the original program.

In John F. Kennedy's 1961 inaugural address :

"Ask not what your country can do for you -- ask what you can do for your country."

17 words, made up of 61 letters, 16 spaces, one dash and one period : total file size of 79 units.

"ask", "what", "your", "country", "can", "do", "for", "you" appear twice

Source: http://computer.howstuffworks.com/file-compression1.htm