The Pulse resembled a glowing red-and-black meteor or "radioactive conch shell" in it's dormant state. It was in the collection of alien collector Henry Parker.
Parker named it and believed it to be a healing device and the only thing that kept him alive despite his ill health. It was not a healing device, but as Owen Harper later learnt, it was actually a reply sent by a race who had received one of Earth's past transmitted attempts in the seventies to discover proof of extra-terrestrial life, containing a message that was "displayed" through a brilliant light show that was generated by the Pulse when it reached full power.
The Pulse started to leak dangerous radiation during its initial activation in Parker's home, and Torchwood stepped in, infiltrating his house and neutralising the device.
Unfortunately, Parker died of a heart-attack moments after his conversation about the Pulse with Owen Harper, who was unable to revive him because he no longer breathed and was therefore unable to perform CPR. (TW: A Day in the Death)