Structures are groups of elements joined together to support loads with stability. There are five main types of structures: frame, shell, arch, mass, and suspension. Frame structures are the most common, composed of long elements joined at the ends, while shell structures use a thin outer layer to spread forces throughout. Arch structures were invented by Romans and use geometry to spread forces to the sides. Mass structures use piled materials for stability, and suspension structures hold elements with cables from tall columns, making them suitable for large bridges.