Roof truss cross section
A truss is an assembly of several pieces of lumber and metal gussets and is typically triangular...Thus forming the shape of the roof. The truss incorporates the rafters, ceiling joists, and soffits into one pre-made unit. They're made by truss companies. They're a time saver in a lot of cases, especially if you aren't an experienced framer.
Conventional framing (or field framing, as in framing in the field) involves using individual pieces of lumber to form the various aspects of the roof, ceiling, and soffits. This is how most homes are constructed.