Radio-Telephone Operators (RTOs) use a Phonetic Alphabet to spell letters in place of just saying the letter itself. By using a word for each letter there is less chance that the person listening will confuse letters. For instance, letters that can easily be confused are "B" and "E". In addition to military use, the phonetic alphabet is used in radio communications around the world by ships, aircraft, and amateur radio operators.
RTO, Task Force Eagle Stabilization Force (SFOR), Balkans, November 2003.
The military relies on the phonetic alphabet to clarify communications. In a military situation, a message that isn't understood correctly can have critical consequences. When an RTO is monitoring a radio transmission in a combat environment, both the quality of the signal and the surrounding noise may make it hard to hear clearly. The phonetic alphabet minimizes the possibility of confusing "C Company" with "G Company" by using the easily distinguishable "Charlie Company" and "Golf Company" instead.
History and Current Military Phonetic Alphabet
Since radio became an important tool of military operations, the US armed forces have used several different phonetic alphabets. That's why World War II movies have characters saying "Able Baker Charlie" while present-day soldiers say "Alpha Bravo Charlie" for the same ABC. Here is a chart of the alphabets used by the U.S. and NATO, along with US Navy signal flags.