Finished boards come in S4S (Surfaced 4 Sides) and are used for a multitude of purposes, including shelving, window ledges, base, and casing.
A Jamb can refer to either a top Jamb or a side Jamb. These are the top and sides of a door or window frame that contact the door or sash. The most common size for interior use is 11/16″ thick by 4-9/16″ wide.