There were at least two models: a compact headset (used aboard Jem'Hadar attack ships) and a larger shoulder-mounted version (used aboard Jem'Hadar battle cruisers). Dominion ships were normally equipped with only two display devices: one for the Vorta commander and the other for the Jem'Hadar First. (DS9: "The Ship")
The virtual display device gave the wearer the ability to view the space around the ship by simply turning his or her head (or as Elim Garak said, "It's like having a viewscreen inside your brain"). (DS9: "Rocks and Shoals")
The device itself was designed to be worn by Vorta and Jem'Hadar; when worn by other species, there may be some side effects. Headaches were one symptom experienced by Humans after wearing the device for an extended period. Cardassians and Bajorans seemed to have no problem wearing the headset. (DS9: "The Ship", "Call to Arms", "A Time to Stand", "Tacking Into the Wind")
Background information Edit
It is possible that, since they were now part of the Dominion, some of these devices were engineered for Cardassian use.
The device is considered as Star Trek predicting technology like Google Glass.