Doors have existed a long time, recorded in Solomon's temple as well as in civilisations throughout history. Materials such as wood, plastic and even stone can make up doors, but they have changed little overall as people's ability to build has improved.
The header is on the top outside of the door frame. It is the piece or pieces running horizontally across the top of the door. The style of the header varies according to the architectural style of the house. There are two headers, one on either side of the door.
The head jamb is the portion of the of the door frame that you see if you look straight up as you pass through the door. It forms the ceiling of the door frame.
The side casing, or leg, is the door frame component that extends horizontally along either side of the door. There are four side casings per door---two legs or side casing pieces on the front side and back side of the door.
The side jamb is on either side on the inside of the door frame. The combination of the side casing and side jamb encases the door frame---the front, back and inside. Near the bottom of the side jambs are dados, or notches, into which the sill slides. At the tops of the side jambs are side jamb lugs, which are hidden inside the opening in which the door is installed. They help to fill the opening and provide some stability to the door frame.
The sill, also known as the threshold, forms the transition between the door and the floor on either side of the door. The sill is usually rounded and angled toward the floor on either side.