Specifically, CTer, here, means someone who believes that there was a successful conspiracy to murder President Kennedy.
In general, CTer means someone who believes in any large conspiracy that successfully kept a secret for a long time, however unlikely this would be. Examples are:
** A worldwide Jewish conspiracy to do evil things to non-Jewish races.
** illuminati
** Freemason?s
** Apollo Moon Landing Hoax
** UFO coverup
A sign of such a conspiracy theory is if the proponents do not provide a list of all the people involved because the list would be too large and make it too obvious that they believe in a large conspiracy.