A hologrid was the technology that covered the surface of the walls of a holodeck.
The hologrid on Galaxy-class starships was made up of yellow squared lines on a black surface. In 2370, while the USS Enterprise-D was transporting the Boraalans from their decimated homeworld to their new planet, various ship-wide malfunctions caused the hologrid to become visible in certain places, such as water and the sky. Fortunately, this civilization was not technologically-advanced, so they thought of the disturbances as omens that they were moving in the right direction. (TNG: "Homeward")
On Intrepid-class ships, such as the USS Voyager, the hologrid had been upgraded and had the appearance of silver walls and thin scaffoldings holding holoemitters in place on the walls. This configuration of hologrid was also seen on the ship that Section 31 operated under the orders of Luther Sloan. (DS9: "Inquisition")
On Deep Space 9, Quark owned a number of holosuites. The configuration of the hologrids inside them was very different from Starfleet models. They had visible emitters, and each one was individually accessible. The holosuites run by Quark were installed during the Cardassian Occupation, and may have been Cardassian in design. (DS9: "The Wire") In 2375, Ensign Nog attempted to re-activate Vic Fontaine's program by directly accessing the hologrid controls, but was unsuccessful. (DS9: "It's Only a Paper Moon")