Joists are horizontal structural members used in building construction to support floors and ceilings. They are typically made of wood, steel, or engineered materials and are spaced evenly to distribute weight. Joists help create a stable framework that allows for the installation of flooring materials and provides structural integrity to a building. In residential construction, floor joists are commonly found in basements and upper levels, while ceiling joists support the ceiling below. Proper installation and spacing of joists are crucial for ensuring safety and preventing sagging or structural failure over time.