Cantilever bridge
This is employed mostly to overcome construction difficulties where it is not practical to temporarily support the bridge from below during construction. An advantage of the cantilever is that it can be constructed by working only from the support caissons - this is done by building each side of the cantilever in sychronization to ensure the balance of the structure. Typically, two or more such cantilevers upon completion will be securely anchored to massive supports at their outer spans to resist the inward tipping of the cantilevers, with a substantial gap between the two cantilevers. A prefabricated central span will then be lifted into place using cables. One of the most famous of such bridges is the Firth of Forth bridge in Scotland, notable for its innovative use of tubular structural elements. The eastern span of the San Francisco Bay Bridge uses this structure, soon to be replaced by a more modern (and suposedly more earthquake resistant) span.
Moveable bridges
To allow ships to pass which can not pass under it, a bridge may be constructed such that it (or part of it) can be turned up (drawbridge; either one part or two) or sideways (swing bridge). A third method is that the bridge deck is lifted while staying horizontal (lift bridge or lifting bridge). (Alternatively, if road traffic is very light, a transporter bridge may be used.)
The tilting Gateshead Millennium Bridge spanning the river Tyne between Gateshead on the south bank and Newcastle upon Tyne on the north (see image below), is a pedestrian bridge with two huge hydraulic rams at each side that tilt the structure back allowing small boats, etc. through.
For small bridges these movements may be enabled without the need for an engine. Some bridges are operated by the users, especially those with a boat, others by a bridge-person, sometimes remotely using video-cameras and loudspeakers.
There are often traffic lights for the road and water traffic, and moving barriers for the road traffic.
Smaller moveable bridges, called jetways, are used in airports to allow passengers to cross the variable distances and heights between the terminal building and aircraft of varying sizes.
See also
Works of art
featuring bridges or using a bridge metaphor
Related topics
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A bridge is a structure built to span a gorge, valley, 
spanning the River Avon in 
Modern vehicle-carrying suspension bridges use towers to support the main load bearing cables. These cables must be securely anchored at each end of the bridge, since any load applied to the bridge is transformed into a tension in these main cables. The roadway is supported by vertical suspender cables. This type of bridge is the only practical type usable for longer spans, where it is impractical or hazzardous to maritime traffic to add central supports. This kind of bridge is particularly pleasing to the visual senses, with the most beautiful of the type being generally acknowleged as the 
Cable-stayed bridges are relatively modern. The design uses a number of individual cables from the tower to the roadway. These do not arc (except under their own weight) but proceed in straight lines to multiple attachment points along the roadway. Bridges are usually composed of pairs of towers, but the method is not restricted to this form. Triple tower bridges have been constructed and a single tower bridge has been designed (but not yet constructed) as the main replacement span for the easter section of the San Francisco-Oakland Bay Bridge. Perhaps the most esthetically pleasing single tower bridge is the recently completed (opened July 4, 2004)