A bridge is a structure built to span a gorge, valley, road, railroad track, river or other body of water, or any other physical obstacle.
The purpose of a bridge is to allow passage of people, cars, trains or ships
over an obstacle.
A moveable bridge gives space for tall items such as ship's masts to pass through, or may be used to span distances that are variable in height or distance.
The Oxford English Dictionary traces the origin of the word bridge to an Old Norse word brygga, meaning "landing stage, gangway, or movable pier".
spanning the River Avon in Bristol, England]]
A bridge is usually either designed for trains, pedestrian or road traffic or may carry a pipeline or waterway for water transport or barge traffic. In some cases there may be restrictions in use; for example, it may be a bridge carrying a highway and forbidden for pedestrians and bicycles, or a pedestrian bridge, possibly also for bicycles. A bridge which has a series of spans, typically arches, is called a viaduct. An aqueduct is a kind of bridge that carries water, resembling a viaduct. Sometimes a bridge carries a pipeline only. When a bridge spans a road or railroad track, it is often called an overpass (US) or flyover (UK). A skyway is typically a fully-enclosed bridge between two buildings.
Compared to a log or wood beam bridge, a truss bridge can be much more economical in its use of materials, partcularly when wood truss members are combined with metal fittings to transfer tension through the structure. Trusses can carry heavier loads over longer spans using shorter elements than can log bridges. Conceptually, simple trusses are considered to be pin-joined triangulated structures, where the engineering analysis of the structure depends upon the assumption that there is no bending stress placed upon the truss members. Such structures may be analysed using simple methods. The assumption of bending stresses require a more sophisticated analysis, yet well within the capablilities of pre-computer engineering. Since most early bridges of this type were constructed largely of wood, it is typical that they are either formed from treated wood (typically using creosote or are protected with a roof and enclosed in building-like siding, forming a covered bridge. There is a substantial variation in the design of trusses, but all use the principle of alternating tension and compression elements (some elements perform both tasks under dynamic loading). Where elements are known by engineering analysis to carry only tension, they may be made of slender steel rods. The upper chord of a truss is generally in compression, while the lower is in tension.
Bridges generally use a pair of trusses joined with diagonally braced horizontal elements to form a box structure. The roadbed may form part of the upper or lower elements or may be suspended somewhere in the middle. Where a bridge must span an canyon at an angle the side trusses may be offset. This is usually seen where a highway or railroad through a canyon is carried over a stream to take advantage of more favorable grading or construction opportunities presented by the opposite bank.
Truss bridges may be made from almost any material with sufficient rigidity and strength, including peeled logs, wood beams, reenforced concrete, and metal beam structural elements.
The concepts of the truss are used in other types of bridges and in components of bridges such as the deck structures of some suspension bridges.
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.
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) 