Truss bridge is a type of bridge whose main element is a truss which is a structure of connected elements that form triangular units. Truss is used because it is a very rigid structure and it transfers the load from a single point to a much wider area. Truss bridges appeared very early in the history of modern bridges and are economic to construct because they use materials efficiently.
Before Industrial revolution (19th century), almost all bridges in use were made of stone. But wood and iron can resist tension and compression better and stone and United States had much wood so they made many wooden bridges in those times and most of them were truss bridges. Town's lattice truss, a very simple variant of truss, was patented in 1820. First half of 19th century saw very few truss bridges made of iron although the first patent for an iron truss bride was issued to Squire Whipple in 1841. But metal slowly started to replace wood, and wrought iron bridges started appearing in the U.S. in the 1870s only to be replaced by steel in 1880s and 1890s. In time some places (like Pennsylvania) continued building truss bridges for long spans well into 1930s, while other (like Michigan) started building standard plan concrete girder and beam bridges.
From the first truss bridge, engineers experimented with different forms of truss bridges trying to find better shape and the one that will suit them for the particular problems. Because of that we have today many forms of truss bridges. Truss bridge can have deck (roadbed) on top (deck truss), in the middle (through truss), or at the bottom of the truss. If the sides of the truss extend above the roadbed but are not connected, it is called a pony truss or half-through truss.
Here are some more common variants of truss design for bridges: