Military time is a four digit number from 0000 (midnight) to 2359 (one minute to midnight). Hour times (eg: 6am) are pronounced 'oh-six-hundred'. Other times (eg: 11:30pm) are pronounced 'twenty-three-thirty'.
It's useful in programming because you don't have to worry about parsing the colon (:). Just take the integer and modulus after dividing by 100 and you have the hours and minutes.