As the FAQ points out, covered bridges are roofed and covered mainly to protect the superstructure, the truss, from rot. The main structural types of covered bridges are those with different truss types. In many cases, all bridges in a particular county use the same truss -- having been built by the same builders, or, at least, approved by the same county services.
Most sources classify the bridge trusses from the simplest to more complex ones more or less as follows:
