To understand how a section works, it is good to show one adjacent to an exterior elevation. Both of these drawings of Frank Lloyd Wright's Robie House look in the same direction, but the elevation is seen from outside the house, while the section is seen from within. Just imagine slicing the building apart like a cake and seeing the layers (floors, roofs, rooms) of which it is made.
Building sections are typically generated by extruding information from floor plans; this applies to hand drafting as well as to computer drafting and modeling. This section of Vinci | Hamp's Mid-North Residence is "cut" from the plan just below the horizontal midpoint of the rectangular plan, extending from the stair on the far right to the window bay just above the front door at the far left.
Building sections serve many purposes: showing relationships between spaces, broadly describing the structural system of a building and expressing a building's form. The latter is pronounced in this section of WA Design's Berkeley Courtyard House, in which angled roofs define indoor and outdoor spaces.
More detailed sections that describe construction, wall assemblies, materials and other ingredients are called wall sections. They are drawn at a larger scale than building sections and are typically drawn after the relationships explored in the building section have been defined and set.