The original Transformers cartoon featured the Space Bridge, which created a subspace tunnel from one point to another — typically, from an Earth station to Cybertron. The Earth station resembled a giant ring lying down, with an opening gate. When activated, the gate would close and sections of the ring would light up, the light spinning until a subspace tunnel was opened and whatever was in the ring was pulled in. The Cybertron receiving station was built into a large tower. Only one Space Bridge was ever shown, and it was under Decepticon control; but one station existing on Cybertron may be a hint that there were other Space Bridge stations on Cybertron. It was a key plot device in the first two seasons, phased out afterwards in favor of more standard space travel.
Unlike some examples (see Stargate above), matter is not broken down when sent through the Space Bridge — the objects stay intact inside the subspace tunnel.
In the live-action movie series, specifically in Transformers: Revenge of the Fallen, the Space Bridge is portrayed differently. The idea and function is basically the same: it is a subspace tunnel system used by the Decepticons to transport matter, intact, across distances instantly. The difference here is that instead of a facility which cannot move, it appears to be an ability possessed by the Decepticons in general, or some device they carry with them, not unlike the Boom Tubes created by Motherboxes. Indeed the idea is quite similar. In this incarnation when the Space Bridge is activated there is a huge "bang" or explosion, probably air being drawn in and displaced when the transition occurs, and with a flash of light objects are transported elsewhere. The range of this version of the Space Bridge is unknown, but it allows several people, including a robot the size of an SR-71 spy plane, to teleport across the globe instantly.
Also like the previous incarnation, this portable Space Bridge allows objects to remain intact and indeed interact with one another—such as when Sam was injured during transit.
A third version is seen in Transformers: Dark of the Moon, as a creation of the former Autobot ruler and "Einstein" of Cybertron, Sentinel Prime. This version was much different than the Decepticon version but, ironically, almost the same as the original cartoon incarnation: using immobile devices called "Pillars" situated in a ring shape, it would establish a subspace tunnel between two points in space time, described by Sentinel Prime as using some kind of new physics principal that defy what humanity understands (or more approriately, think we understand) about the universe, to bend space-time so that two points contact. In effect, it is the same principal as a wormhole, only it can be artificially activated and used repeatedly.
Hundreds of Pillars were created by Sentinel Prime and transported to Earth on his space battleship, the Ark, which was damaged in transit to FTL speed and crashed on the moon in the 1960s. The Decepticons attempted to gain control of this model of Space Bridge technology to teleport Cybertron (depicted here as a huge Jupiter Brain-like construct vastly larger than Earth) into our star system and cause it to collide with Earth. Interestingly it operates the same as both previous kinds of space-time bridge system--namely, matter remains intact and functional during the transmission, implying it is the same technology as the previous movie's incarnation only more advanced.
In the comics this is explained in detail: the Sentinel Prime's subspace bridge was a more sophisticated and powerful version of the Decepticon one, which was discontinued after some time because it was too inefficient, compared to the third-model Space Bridge which can transport anything from starships to armies and even whole worlds instead of "just" a few dozen tons of matter across space-time.
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