A structure is defined as a system of interconnected members designed to support or transfer forces and successfully withstand the loads acting on them. The internal forces of a structure can be determined by decomposing the structure and analyzing the free-body diagrams of the individual members or of a combination of members. This helps in understanding the structural elements' behavior and ensuring that the structure is stable and can withstand the subjected loads.

There are three main categories of structures: trusses, frames, and machines. Trusses are stationary structures that remain rigid when subjected to force. The members of a truss are straight elements connected by joints at the ends of each one. Each member of a truss has two equal and opposite forces acting along it. Trusses are commonly used in bridges, roofs, and towers.

Frames are rigid, stationary structures designed to support loads. Members in frames may experience both axial forces and bending moments. However, at least one member of a frame always has three or more forces acting on it, and at least one of those forces is not directed along the axis of the member. Frames are commonly used in buildings and industrial structures, such as cranes and gantries.

Machines are structures with moving parts designed to transmit and modify the effects of forces. They involve mechanisms that allow for motion or force transformation. At least one multi-force member should be present in a machine. Machines are commonly used in tools, such as presses, and vehicles, like cars and planes.