A tailhook is almost always found on naval aircraft of a government's military. It is a hook near the back of the plane that grabs the wires strung across an aircraft carrier's deck.
I have not heard of a 707 having a tailhook.
http://en.wikipedia.org/wiki/Tailhook
Within FS, there is a tailhook command that allow naval addons to deploy their own hooks. Some other addon aircraft simply use this command to enable other, unrelated operations.