This frequency is part of the US Air Force HF-GCS (High Frequency Global Communications System). All US military branches can and do use this system, but it is an Air Force system. Other common freqs to watch are 4724, 6739, 8992, 11175, 13200, and 15016 kHz. Most frequently a message is simulcast on all of those frequencies at the same time.
The most common message format is what hobbiest call an "EAM". EAM stands for Emergency Action Message. In fact all of the messages heard are not EAMs, but it is easiest jsut to lump them together. Also, it is kind of impossible to tell the difference
It is often possible to hear 30 or more such messages in a day.
Another message format heard is the "Skyking" message, you will recognize it the first time you hear it.
Mutiple transmitters are located around the World on each frequency, and they can all be selected to transmit at the same time with the same message, so despite the fact you might hear a message sign off as "Andrews" that does not mean the transmitter you are hearing is actually located at Andrews AFB.