A wall stud is a vertical framing member in a building's wall of smaller cross section than a post. The wall stud is a very important part of your house. It is located in the interior of your house and acts as a frame for the drywall.
Floor System (Floor Joist) - Wood Framed
A floor system is made up of joists, sub-flooring,finished flooring, etc. The importance of the floor system is so that your floor is stable and it doesn't just collapse one day. It is located beneath all of the floor in your home.
Roof System (Rafter or Truss) - Wood Framed
A roof system is very important in every house because without it your house would be exposed to all types of outdoors. The roof system is located, of course, on the top your house. It acts as a system to keep out all the nature from your house, and if not properly built can collapse.