Military Alphabet Chart: NATO & Western Union Standards
Also called a spelling alphabet or a phonetic alphabet, (not to be confused with the entirely different International Phonetic Alphabet,) this is a system of assigning to each letter of the alphabet a word that begins with that letter. This way, if something has to be spelled over a radio, telephone, etc. there is much less chance of the wrong information being transmitted.
The military, police departments and radio operators all make frequent use of this. Phone-based customer service and technical support also use it, but with more informal construction (any word will do), for the same reasons. (The formal ones specifically pick words themselves which all sound distinct from all the others with poor sound quality.) The most common alphabets are shown below, from A to Z.