The viewscreen is a feature common to the bridges of all Federation starships and most alien starships in Star Trek. The viewscreen occupies most of one wall of the bridge, allowing essentially the entire bridge crew to clearly see whatever it displays.
The viewscreen is a high-tech computer monitor, displaying images rendered by the computer based on sensor data, communication transmissions, or other information stored in the ships data stores. The viewscreen is not a window and displays nothing if the ship's computer is not functioning (except in the 2009 Star Trek movie, in which the viewscreen is actually a window). The viewscreen will display false images if the ship's sensors have somehow been "spoofed" with false information.