A mirror is an item with a highly polished and reflective surface. Most mirrors were flat, although some had a curved surface to focus light or to distort the reflected image.
The most common use for a mirror was personal grooming. The holding cells in the brig and crew quarters aboard the USS Enterprise-D and Deep Space 9 featured a restroom and a mirror for such a purpose. An individual standing in front of the mirror and could see his reflection which facilitated grooming. Other uses included mirrors as components in telescopes, lasers, and cameras. (TNG: "The Hunted"; DS9: "Vortex"; Star Trek: First Contact; ENT: "Two Days and Two Nights")
One notable mirror was a intricate mechanism that allowed Trelane to perform a wide variety of superhuman feats. James T. Kirk shot the mirror and the intricate machinery behind the mirror was destroyed collapsing much of Trelane's creation, enabling the landing party to escape and flee Gothos. (TOS: "The Squire of Gothos")
Beyond technical and personal use, mirrors were sometimes used as simple decorations in architectural design. The San Francisco bar was a bar on Earth in San Francisco in the 23rd century which featured mirrors in this function. In 2285, Doctor Leonard McCoy visited this bar to hire a transport ship to carry him to the planet Genesis in the Mutara sector. The bar had a lounge like atmosphere complete with tables, holographic parlor games, lava lamps, plants, and of course, mirrors. (Star Trek III: The Search for Spock)
Often objects that were not necessarily mirrors were considered "mirror like" because they appeared to be reflections of a "normal" object. An example of this was the "mirror universe" which was the informal name for the parallel universe first recorded as visited by James T. Kirk and several officers from the USS Enterprise in 2267. This mirror universe coexists with our universe on another dimensional plane. The universe was so named because many people and places seemed to be the reflection of their "normal" selves from "our" universe, but with numerous "good" aspects now "evil", and vice versa. (TOS: "Mirror, Mirror")