What is it:

The tail boom in some planes houses a radar, so as to send and recieve radar waves or signals from the rear too. This advantage is not present in most aircraft. However, these very often have radar recievers, so as to detect the radar of incoming missiles. The automated features in some planes automatically release flares and chaffs to ward it off.
In many cases of larger aircrafts, the tail boom is often a little long and bulging, and this is because of two reasons.
One reason is that it has a radar system here too. This is seen in the BAe Nimrod, which has been modified into an AWACS aircraft!
The other reason is that it contains the MAD systems to detect submarines below the surface of the seas!