The bridge of a ship is the room or area of the ship from which the ship is controlled. It typically has stations that that can either directly or indirectly control communications, sensors, navigation, and weapons. On a modern warship, the bridge is typically located in the superstructure, high above the waterline, to give the occupants a good field of view. The bridge is also typically armored against attack at least as well as any other part of the superstructure.
A modern ship will also typically have a secondary command and control center located in another part of the ship. This C&C will also have appropriate workstations and will be manned in any combat situation so it can assume control of the ship if the primary bridge is destroyed or disabled.
Bridges in Science Fiction
The bridge of a Federation starship is typically located at the top (most dorsal point) of the primary hull. There is usually also an auxiliary bridge in the secondary hull, near the main engineering compartment; in Galaxy-class starships, the secondary bridge is known as the "battle bridge", as it is used to command the engineering section when it separates from the saucer section, a situation originally intended to occur only if combat was expected. In practice, the auxiliary bridge of a Federation starship is seldom manned, even when the ship is in combat.
On multiple occasions, intruders have seized control of Federation starships by gaining control of the auxiliary bridge. The control systems on the auxiliary bridge can actually override the controls on the main bridge, even when the ship's commanding officers are in control of the main bridge. The reason for this control priority situation is unknown.
The main bridge of an Imperial Star Destroyer is located high in the dorsal superstructure of the ship. It is unclear what auxiliary control stations exist in Imperial starships, but when the main bridge of the Super Star Destroyer Executor was destroyed by a kamikaze attack, the crew were unable to regain navigational control before the ship crashed into the Death Star.