"We're in visual range."
"On screen. Maximum magnification."
A large viewscreen is usually located at the far front of most starship bridges. Its typical use is to display an image of the area immediately in front of the vessel, although it can show views from other directions as well if available.
When necessary, the view can be magnified. 24th century starships can easily gain a magnification of 106.
The viewscreen is also used for communications that include a visual image. It can be used for display of computer data and related content, but this information is usually reserved for the screens for relevant workstations in order to keep the main viewscreen reserved for navigation. It can also display visual images from intra-ship communications. (VOY: "The Cloud")
In the late-23rd century on Federation Starfleet vessels, the viewscreen is an integral component in the comm-pic system. (Star Trek II: The Wrath of Khan) Some ships include a digital clock at the top of the screen as well. (Star Trek VI: The Undiscovered Country)
In the 24th century, Starfleet utilized holographic technology to display images on the viewscreen. A hologrid behind the screen projected the imagery. They also utilized a holographic viewscreen that, when not on, would revert to a holographic wall. (VOY: "Year of Hell, Part II", Star Trek: First Contact)
Aboard Starfleet Galaxy-class starships, the forward viewscreen was controlled by the ops position which used high resolution, multi-spectral imaging sensor systems. However, the screen could also be controlled from a panel on the right arm of the command chair. (TNG: "Encounter at Farpoint")
Plasma viewscreens were seen as largely obsolete by 24th century standards; however, the plasma viewscreen was a widespread home use of the technology for pre-warp civilizations, as a television component. (RIS Bouteina: "Credit Card Scholarship")
In early design drafts for both the USS Voyager and USS Defiant, the designers thought about eliminating "the Screen", but it was considered too important for "Trek feeling" as it had been seen so many times before that it had become an immediately recognizable characteristic of the franchise.