(covers information from several alternate timelines)
A viewscreen (or main viewer) was an audio/visual device used as early as the 2150s aboard starships, space stations, and at planetary facilities by space-faring organizations including Starfleet, the Borg Collective, the Cardassian Union, the Klingon Empire, and the Romulan Star Empire.
Generally consisting of a large screen located on the bridge of a starship (or operations center of a space station or starbase), the viewscreen was an almost universal facet of space exploration and colonization dating as far back as the 22nd century, surviving well into the 23rd and 24th centuries.
Typically used to display images of the area immediately around or in front of a starship, the viewscreen could provide views from all directions, as well as call up data from the library computer. It was also essential in ship-to-ship communication, allowing face-to-face conference if so desired, utilizing subspace and other communications systems. Visual contact, however, could only be achieved when in visual range. Intra-ship communications were also possible, though the main viewer was rarely used for this function. (Star Trek V: The Final Frontier; TNG: "Ship in a Bottle"; VOY: "The Cloud")
When necessary, the image on the viewscreen could be magnified - 24th century starships easily gaining a magnification of 106. The image could also be augmented, with the ship's computer displaying extrapolated images or graphics displaying sensor data. (Star Trek VI: The Undiscovered Country; ENT: "Broken Bow")
Installed as early as 2151 aboard Starfleet vessels including NX-class starships, the viewscreen was located on the front bulkhead of the main bridge. Rectangular in shape, the main viewer aboard these vessels provided a wide screen view of the space around the ship, as well as visual communications. They were capable of processing multiple inputs simultaneously, as was sometimes seen when two or more parties wished to have a joint conversation with Enterprise.
In the year 2152, the crew of Enterprise NX-01 mounted a sophisticated sensor on the grappler arm allowing their sensors to detect and display onscreen cloaked Suliban and Romulan vessels. (ENT: "Shockwave", "Minefield")
Returning to the same basic shape of the 22nd century viewscreen, the main viewer utilized aboard such 23rd century Starfleet vessels as the Constitution-class USS Enterprise was mounted at the front of the main bridge and was generally rectangular in shape.
Though a smaller viewscreen (utilized as early as 2254) would be supplanted by a larger one in 2266, the viewscreen aboard the Constitution-class vessel generally displayed images with a blue outline and featured a number of controls mounted to the left and right of the monitor. Capable of the same function as previous versions, this viewscreen model could also be used to display sensor data from within the starship, call up cross sectional diagrams of the ship itself, and display data re-routed from other stations. (TOS: "The Cage", "The Menagerie, Part II", "Where No Man Has Gone Before"; ENT: "In a Mirror, Darkly, Part II"; TOS: "Spock's Brain")
In the alternate reality caused by Nero, the USS Enterprise's viewscreen continued the Kelvin-type design. Its viewscreen was a single window the height of a crewmember, that could have images projected on it. (Star Trek)
During its refit in 2270, a new viewscreen was placed on the bridge of the Enterprise, this one much larger and sometimes utilized to display alert status. By 2293, refit Constitution-class starships incorporated a digital clock mounted at the top of the screen. When the ship was moored in spacedock, the inactive viewscreen displayed a generic graphic. (Star Trek: The Motion Picture; Star Trek VI: The Undiscovered Country)
While not projecting solid holographic images, the viewscreen installed on the main bridge of such vessels as the USS Enterprise-D displayed three-dimensional images, as though observing the image with the naked eye.
Larger than that of the Constitution- or Excelsior-class starships, the viewscreen aboard the Galaxy-class starship featured touch-sensitive controls at the bottom of the screen. Using high resolution, multi-spectral imaging sensor systems and could also be controlled from a panel on the right arm of the command chair or at the ops or tactical stations. (TNG: "Encounter at Farpoint")
Slightly smaller in size, the Intrepid-class viewscreen also differed from those of the Galaxy-class in that a hologrid was present behind the displayed image. When damaged or deactivated, a hologrid, much like that in the ship's holodeck, appeared on the bulkhead. (VOY: "Year of Hell, Part II")
One of the more unusual viewscreen designs of the 24th century, the main viewer aboard the Sovereign-class USS Enterprise-E utilized a holographic image projected onto the front bulkhead of the main bridge.
While most viewscreens aboard Federation vessels of the era did use holographic technology, the images they displayed were generally projected within a clearly defined screen area. Not so aboard the Enterprise-E, with the viewscreen appearing from only a small area of projection systems near the floor of the forward bulkhead. When deactivated, the image projected disappeared, leaving only a blank wall in its place. (Star Trek: First Contact)
By 2375, however, such technology was replaced with a standard format viewscreen installed aboard the Enterprise-E, lasting through 2379 until the forward section of the bridge was destroyed during the Battle of the Bassen Rift. (Star Trek: Insurrection; Star Trek Nemesis)
Borg cubes were equipped with special viewscreen technology, projecting images onto a free-floating cube-shaped viewer somewhere within the ship.
During the 24th century, starships and space stations constructed by the Cardassian Union utilized unique, holographically projected viewscreens.
Installed aboard Galor-class warships and Terok and Empok Nor-type stations, these Cardassian viewers consisted of hollow, oval-shaped frames. Images were projected into these frames, then disappeared when the viewer was deactivated.
Jem'Hadar ships eschewed traditional viewscreens for a virtual display device. Instead of occupying the forward wall of the bridge, the "viewscreen" was in fact a display within a headset worn by the navigator. The device was not well-tolerated by Humans, who could typically wear it only for short periods. Cardassians, by contrast, were shown to have similar tolerances to Jem'Hadar and Vorta.
Performing the same basic function as their Federation counterparts, 23rd century Klingon viewscreens aboard such ships as the IKS Amar were able to display tactical information in graphic and photographic formats. (Star Trek: The Motion Picture; Star Trek IV: The Voyage Home; ENT: "Sleeping Dogs")
Consisting of screens smaller than those used by the Federation and Klingon Empire, Romulan viewscreens (like that of the IRW Khazara) were rounded-off square shapes, accented with green rectangles at the top of the monitor.
In early design drafts for both the USS Voyager and the USS Defiant, the designers contemplated eliminating the viewscreen entirely. Attempts such as the holo-communicator in DS9 and the suspended-in-mid-air viewer in First Contact were made, but ultimately gave way to tradition. The viewscreen was considered an important staple of the Star Trek universe.