In the fictional Babylon 5 universe, a Jump Gate is a static emplacement that creates an energy vortex, called a Jump Point, to allow vessels to travel to and from hyperspace for interstellar travel. In the Babylon 5 universe, hyperspace is a dimension running parallel to normal space, in which the distances between points are radically shorter. Jump gates also have beacons that allow vessels in hyperspace to lock on to a beacon and avoid getting lost. Amongst the "younger" races of the galaxy, jump gates are a vital part of interstellar travel.
For most ships, a jump gate is required for interstellar travel. While the more advanced races - such as the First Ones - are able to fit jump point generators on to even its smallest vessels, for younger races the energy cost means that only the largest military vessels have the capability to create their own jump points. The White Star ships used by Babylon 5's allies, and later by the Interstellar Alliance, are amongst the smallest such vessels.
The jump gates are also vital for the beacon signals that they provide, as there are no navigational references within hyperspace itself. Each jump gate broadcasts a beacon signal into hyperspace. The signal itself identifies its position in normal space, and following the beacon leads one to the corresponding point in hyperspace; this bi-spatial junction is called a jump intersection. Ships navigate by tracking these radio-like beacons, and these beacons allow ships to travel through hyperspace without getting lost and drifting so deep into hyperspace that they become permanently lost. If enough jump gates were destroyed the entire jump gate / beacon network would fall apart. Vessels would then be unable to make their way through hyperspace. For that reason jump gates are usually considered neutral territory, and it's considered very uncivilized to attack them.
A jump gate is typically made of four long independent cylinders that form an ellipse. Some races use different configurations (a Narn jump gate employs three cylinders). When a jump gate is activated, an energy pulse runs down each of the cylinders, until it reaches the end of the cylinder. It then separates from the cylinder, and travels out to a point where it meets with the other energy pulses. The energy pulses together provide enough energy to create the vortex. The vortex will be yellow if a ship will be entering the point and going into hyperspace, and will be blue if a vessel is exiting hyperspace. The jump gate will provide the energy to keep the jump point open until a vessel has completed transit.
A vessel wishing to enter a jump gate will usually send a signal to the jump gate to activate the gate. If a private vessel is entering hyperspace from a gate, a fee is charged for the use of the gate. If for whatever reason those in control of the gate wish to keep a certain vessel from entering hyperspace, the computers on the gate can be programmed to reject an activation signal from that vessel. However, whatever controls or fees may be in place for entering hyperspace, the general rule is to allow any vessel to exit hyperspace through the gate freely and without charge. Since the jump gate represents the only way most vessels can return to normal space, they are vital escape routes in emergencies. If a ship's power failed the ship could become forever lost in hyperspace without such an exit. Regardless of circumstances, the gate's computers do record each ship that enters or exits the gate, and such records can be used to establish presence in legal settings.
Larger ships that have their own jump point engines will normally use a jump gate if one is available. This is because the onboard jump point generators require a large amount of energy to operate, and using a jump gate saves energy. However if such a vessel wanted to keep its arrival off the official records, it would then use its own generators to keep a record from being made by a gate.
A jump gate can be turned on or off, but it takes quite some time for the jump gate generators to power up or power down. To either power them up or down too quickly would most likely result in the gate destroying itself in a large explosion. The cylinders are also movable, if an exceptionally large vessel needs to use the gate the cylinders would be moved to accommodate this vessel.
Vessels equipped with their own jump point generators avoid opening their own jump point within an active jump gate; to do so would destroy the jump gate and any nearby vessels. Earth Force did experiment with the idea during the Earth-Minbari War. After learning about the consequences (and the fact that no Earth ship could escape those consequences), they labeled their failure the "Bonehead Maneuver" (referring to the Earth slang for an idiot rather than anything pertaining to the Minbari). Captain John Sheridan uses the maneuver with the first White Star to destroy a Shadow vessel in the episode Matters of Honor: the White Star actually being fast enough to escape the explosion.