The PBS television show "Building Big" explains that a continuous-span bridge is simply a set of single-beam bridges linked together to form one long bridge. A single-beam bridge is a horizontal piece of material held up on each end by a support, such as a plank laid across a ditch.
With few exceptions, the length of a single-beam bridge is only 250 feet or fewer. When connecting many bridges together, however, a continuous-span bridge can go for miles. PBS gives the example of the Chesapeake Bay Bridge-Tunnel, which spans 79,200 feet and contains 5,000 pier supports, each of which is 100 feet or fewer apart.