A stud is the board that is used to create the vertical sections of the frame work. These are often 2×4 pine boards that are put into the major frame work at distances of approximately two feet apart, depending on the structure. The studs can also be made of metal or concrete based on the type of building that is being discussed. The purpose of the stud is to hold up the wallboard, typically sheetrock. The wallboard is nailed or screwed to the studs at even intervals. This provides you with an excellent means of keeping the wall board secure, allowing for a space for insulation between the wall boards and giving a place to safely tuck away electrical wiring and plumbing throughout the structure.
If you are putting an addition on your home the contractor will likely be doing some additional framing. This framing should include studs of the appropriate size and type. They will likely drill holes through the studs in order to run the new electrical through the walls. This is most often done after the insulation is put into the wall. The stud is the main support for all wall board that is installed and will help the contractor to create the look you want. If there is a doorway, pass through or window the professional will work the studs up to these areas and attach them to the sills or framing around the area.