A Holodeck or Holosuite is a room specifically equipped with holographic emitters capable of projecting photons and force fields to create the physical illusion of matter. As such they can simulate an object, environment or narrative scenario in a completely immersive environment. They are also capable of creating sentient Photonic beings... see this link.
New missions have appeared on Star Trek Online, the graphics are getting better than previous years, the intro sequences are great, especially in the mission "Beneath the Skin". Here you see me exiting the holodeck...